Semiconductor optical amplifier

ABSTRACT

A semiconductor optical amplifier includes: a laminated structure sequentially including a first compound semiconductor layer composed of GaN compound semiconductor and having a first conductivity type, a third compound semiconductor layer having a light amplification region composed of GaN compound semiconductor, and a second compound semiconductor layer composed of GaN compound semiconductor and having a second conductivity type; a second electrode formed on the second compound semiconductor layer; and a first electrode electrically connected to the first compound semiconductor layer. The laminated structure has a ridge stripe structure. When widths of the ridge stripe structure in a light output end face and the ridge stripe structure in a light incident end face are respectively W out , and W in , W out &gt;W in  is satisfied. A carrier non-injection region is provided in an internal region of the laminated structure from the light output end face along an axis line of the semiconductor optical amplifier.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor optical amplifier.

2. Description of the Related Art

In these days, in the advanced scientific region researches using laserlight with the pulse time on the attosecond time scale or on thefemtosecond time scale, the ultrashort pulse and ultrahigh power laseris actively used. Further, the high power and ultrashort pulse laserdiode device that is composed of GaN compound semiconductor and that haslight emitting wavelength of 405 nm band has been expected to be a lightsource for a volumetric optical disc system expected as a nextgeneration optical disc system displacing the blu-ray optical discsystem or has been expected to be a light source demanded in the medicalfield, the bio imaging field and the like.

As the ultrashort pulse and ultrahigh power laser, for example,titanium/sapphire laser is known. Such a titanium/sapphire laser is anexpensive and large solid laser light source, which is a main factor toinhibit spread of the technology. If the ultrashort pulse and ultrahighpower laser is realized with the use of a laser diode or a laser diodedevice, significant miniaturization, price reduction, and high stabilityare able to be realized, which is expected to become a breakthrough forpromoting its wide usage in these fields.

Meanwhile, short pulsation of the laser diode device has been activelyresearched since 1960s in the communication system field. As a method ofgenerating short pulses in the laser diode device, gain switchingmethod, loss switching method (Q switching method), and mode lockingmethod are known. In these methods, high output is pursued by combiningthe laser diode device with a semiconductor amplifier, a nonlinearoptical device, an optical fiber and the like. The mode locking isfurther categorized into active mode locking and passive mode locking.To generate light pulses based on the active mode locking, an externaloscillator is configured by using a mirror or a lens, and further highfrequency (RF) modulation is added to the laser diode device. Meanwhile,in the passive mode locking, light pulses are able to be generated bysimple direct current drive by using a laser diode device having amultiple electrode structure.

In the laser light source, obtaining high power is a big challenge. As ameans for amplifying light from the laser light source, thesemiconductor optical amplifier (SOA) has been keenly examined. Theoptical amplifier is an amplifier that directly amplifies an opticalsignal in a state of light without converting the optical signal to anelectric signal. The optical amplifier has a laser structure withoutresonator, and amplifies incident light by light gain of the amplifier.

In the past, the optical amplifier has been mainly developed for opticalcommunication. Thus, for practical application of the semiconductoroptical amplifier in 405 nm band, very few precedent cases exist. Forexample, based on Japanese Unexamined Patent Application Publication No.5-067845, the semiconductor optical amplifier in 1.5 μm band that usesGaInAsP compound semiconductor and that has a tapered ridge stripestructure has been known. In the technique disclosed in the foregoingJapanese Unexamined Patent Application Publication No. 5-067845, in thesemiconductor optical amplifier, a light guide width is gently extendedin tapered shape from the narrow input-side-light guide satisfyingsingle mode conditions to output-side-light guide. Thereby, mode fieldis expanded along the light guide width to improve maximum output of thesemiconductor optical amplifier.

SUMMARY OF THE INVENTION

However, it becomes clear by studies by the inventors of the inventionas follows. That is, in the semiconductor optical amplifier composed ofGaN compound semiconductor, even if the light guide width on the outputside is widened, the width of an outputted near-field image is notexpanded and is narrower than the light guide width. The foregoing factmay lead to inhibition of increase of the maximum output of thesemiconductor optical amplifier, and instability of laser lightoutputted from the semiconductor optical amplifier.

Accordingly, in the invention, it is firstly desirable to provide asemiconductor optical amplifier composed of GaN compound semiconductorthat is able to retain higher light output. Further, it is secondlydesirable to provide a semiconductor optical amplifier with which thereis no possibility that laser light outputted from the semiconductoroptical amplifier is unstable.

According to a first embodiment to a third embodiment of the inventionto attain the foregoing first and the second objects, there is provideda semiconductor optical amplifier including: a laminated structure inwhich a first compound semiconductor layer that has a first conductivitytype and is composed of GaN compound semiconductor, a third compoundsemiconductor layer that has a light amplification region (carriernon-injection region, gain region) composed of GaN compoundsemiconductor, and a second compound semiconductor layer that has asecond conductivity type different from the first conductivity type andis composed of GaN compound semiconductor are sequentially layered; asecond electrode formed on the second compound semiconductor layer; anda first electrode electrically connected to the first compoundsemiconductor layer, wherein the laminated structure has a ridge stripestructure. When a width of the ridge stripe structure in a light outputend face is W_(out), and a width of the ridge stripe structure in alight incident end face is W_(in), W_(out)>W_(in) is satisfied.

In the semiconductor optical amplifier according to the first embodimentof the invention to attain the foregoing first object, a carriernon-injection region is provided in an internal region of the laminatedstructure from the light output end face along an axis line of thesemiconductor optical amplifier.

In the semiconductor optical amplifier according to the secondembodiment of the invention to attain the foregoing second object, awidth of the second electrode is narrower than the width of the ridgestripe structure.

In the semiconductor optical amplifier according to the third embodimentof the invention to attain the foregoing second object, when a maximumwidth of the ridge stripe structure is W_(max), W_(max)>W_(out) issatisfied.

In the semiconductor optical amplifiers according to the firstembodiment to the third embodiment of the invention, where the width ofthe ridge stripe structure in the light output end face is W_(out), andthe width of the ridge stripe structure in the light incident end faceis W_(in), W_(out)>W_(in) is satisfied. That is, the light guide widthis broadened from the light guide on the light output side having anarrow width satisfying single mode conditions to the light guide on thelight output side having a wide width. Thus, mode field is able to beexpanded according to the light guide width, high light output of thesemiconductor optical amplifier is able to be attained, and laser lightis able to be optically amplified while single lateral mode ismaintained.

Further, in the semiconductor optical amplifier according to the firstembodiment of the invention, the carrier non-injection region isprovided in the internal region of the laminated structure from thelight output end face along the axis line of the semiconductor opticalamplifier. Thus, the width of laser light outputted from the lightoutput end face is able to be broadened. Therefore, higher light outputis able to be attained, and reliability is able to be improved.Meanwhile, in the semiconductor optical amplifier according to thesecond embodiment of the invention, the width of the second electrode isnarrower than the width of the ridge stripe structure. In thesemiconductor optical amplifier according to the third embodiment of theinvention, when the maximum width of the ridge stripe structure isW_(max), W_(max)>W_(out) is satisfied. Thereby, stable lateral modeamplified light is obtained, and there is no possibility that laserlight outputted from the semiconductor optical amplifier becomesunstable.

Other and further objects, features and advantages of the invention willappear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual view of a light output device of a first exampleincluding a semiconductor optical amplifier.

FIG. 2 is a schematic cross sectional view of the semiconductor opticalamplifier where the semiconductor optical amplifier is cut along avirtual vertical plane (YZ plane) including an axis line (Z direction)of the semiconductor optical amplifier of the first example.

FIG. 3 is a schematic cross sectional view of the semiconductor opticalamplifier where the semiconductor optical amplifier is cut along avirtual vertical plane (XY plane) orthogonal to the axis line of thesemiconductor optical amplifier of the first example.

FIG. 4 is a schematic perspective view of the semiconductor opticalamplifier of the first example.

FIG. 5 is a schematic plan view of a ridge stripe structure in thesemiconductor optical amplifier of the first example.

FIGS. 6A and 6B are respectively near-field images of laser lightoutputted from the semiconductor optical amplifier of the first exampleand a semiconductor optical amplifier of Comparative example 1.

FIG. 7 is a conceptual view of a light output device of a second exampleincluding the semiconductor optical amplifier.

FIG. 8 is a schematic cross sectional view of the semiconductor opticalamplifier where the semiconductor optical amplifier is cut along avirtual vertical plane (YZ plane) including an axis line (Z direction)of the semiconductor optical amplifier of the second example.

FIG. 9 is a schematic cross sectional view of the semiconductor opticalamplifier where the semiconductor optical amplifier is cut along avirtual vertical plane (XY plane) orthogonal to the axis line of thesemiconductor optical amplifier of the second example.

FIG. 10 is a schematic end view taken along the direction in which aresonator of a mode locking laser diode device in the second exampleextends.

FIG. 11 is a schematic perspective view of the semiconductor opticalamplifier of the second example.

FIG. 12 is a schematic plan view of a ridge stripe structure in thesemiconductor optical amplifier of the second example.

FIG. 13 is a graph schematically illustrating change of a current flownin the semiconductor optical amplifier in the case where a given valueof voltage is applied to the semiconductor optical amplifier whilemaking laser light enter the semiconductor optical amplifier of thesecond example from the laser light source and XYZ stage is moved in theX direction.

FIG. 14A is a conceptual view of a modification of the light outputdevice of the second example, and FIG. 14B is a conceptual view of amonolithic semiconductor optical amplifier.

FIGS. 15A and 15B are schematic perspective views of semiconductoroptical amplifiers according to a third example and a fourth example.

FIG. 16 is a schematic plan view of a ridge stripe structure of thesemiconductor optical amplifier of the third example illustrated in FIG.15A.

FIGS. 17A and 17B are schematic perspective views of semiconductoroptical amplifiers of modifications of the third example and the fourthexample.

FIG. 18 is a schematic plan view of a ridge stripe structure of thesemiconductor optical amplifier of the modification of the third exampleillustrated in FIG. 17A.

FIGS. 19A and 19B are views respectively and schematically illustratinga system of performing mode locking drive by configuring an externalresonator from the mode locking laser diode device in the second exampleand a mode locking laser diode device in a sixth example.

FIGS. 20A and 20B are views respectively and schematically illustratinga system of performing mode locking drive by configuring an externalresonator from a mode locking laser diode device in a fifth example, andFIG. 20C is a view schematically illustrating a system of performingmode locking drive by using the mode locking laser diode device in thesixth example.

FIG. 21 is a schematic view viewed from above of a ridge section in amode locking laser diode device of a seventh example.

FIGS. 22A and 22B are views respectively and schematically illustratinga system of performing mode locking drive by using a mode locking laserdiode device in an eighth example and a mode locking laser diode devicein a ninth example.

FIG. 23 is a schematic end view taken along a direction in which aresonator of a modification of the mode locking laser diode device inthe second example is extended.

FIG. 24 is a schematic end view taken along a direction in which aresonator of another modification of the mode locking laser diode devicein the second example is extended.

FIGS. 25A and 25B are graphs illustrating reverse bias voltagedependence measurement results of relation between an injection currentand light output (L-I characteristics) in the second example and asecond referential example.

FIGS. 26A and 26B are views illustrating results obtained by measuringlight pulse generated in the second example and the second referentialexample by a streak camera.

FIG. 27 is a graph illustrating result obtained by measuring an electricresistance value between a first section and a second section of asecond electrode of the mode locking laser diode device obtained in thesecond example by four terminal method.

FIGS. 28A and 28B are graphs respectively illustrating results ofmeasuring RF spectrum of the eighth example and an eighth referentialexample.

FIGS. 29A and 29B are schematic partial cross sectional views of asubstrate and the like for explaining a method of manufacturing the modelocking laser diode device in the second example.

FIGS. 30A and 30B are schematic partial cross sectional views of asubstrate and the like for explaining a method of manufacturing the modelocking laser diode device in the second example following FIG. 29B.

FIG. 31 is a schematic partial end view of a substrate and the like forexplaining a method of manufacturing the mode locking laser diode devicein the second example following FIG. 30B.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the invention will be hereinafter described based on examples withreference to the drawings, the invention is not limited to the examples,and various numerical values and materials in the examples areexemplification. The description will be given in the following order:

1. Semiconductor optical amplifiers according to a first embodiment to athird embodiment of the invention and overall description

2. First example (the semiconductor optical amplifier according to thefirst embodiment of the invention)

3. Second example (modification of the first example)

4. Third example (the semiconductor optical amplifiers according to thesecond embodiment and the third embodiment of the invention)

5. Fourth example (modification of the third example)

6. Fifth example (modification of the mode locking laser diode device inthe second example)

7. Sixth example (another modification of the mode locking laser diodedevice in the second example)

8. Seventh example (another modification of the mode locking laser diodedevice in the second example)

9. Eighth example (another modification of the mode locking laser diodedevice in the second example)

10. Ninth example (another modification of the mode locking laser diodedevice in the second example) and others

Semiconductor Optical Amplifiers According to a First Embodiment to aThird Embodiment of the Invention and Overall Description

In the semiconductor optical amplifier according to the first embodimentof the invention, W_(out) may be 5 μm or more. Though the upper limit ofW_(out) is not limited, for example, 4×10² μm can be exemplified as theupper limit of W_(out). Further, in the semiconductor optical amplifieraccording to the first embodiment of the invention, W_(in) may be from1.4 μm to 2.0 μm both inclusive. The foregoing preferred embodiments maybe applied to the semiconductor optical amplifiers according to thesecond embodiment and the third embodiment of the invention.

In the semiconductor optical amplifier according to the secondembodiment of the invention, the value of (width of a secondelectrode)/(width of a ridge stripe structure) is desirably from 0.2 to0.9 both inclusive, and is preferably from 0.6 to 0.9 both inclusive.The width of the second electrode and the width of the ridge stripestructure mean the width of the second electrode and the width of theridge stripe structure obtained where the semiconductor opticalamplifier is cut in a certain virtual plane orthogonal to the axis lineof the semiconductor optical amplifier.

Further, in the semiconductor optical amplifier according to the thirdembodiment of the invention, 0.2≦W_(out)/W_(max)≦0.9 is desirablysatisfied, and 0.5≦W_(out)/W_(max)≦0.9 is preferably satisfied.

In the semiconductor optical amplifiers according to the secondembodiment and the third embodiment of the invention, as in thesemiconductor optical amplifier according to the first embodiment of theinvention, a carrier non-injection region may be provided in an internalregion of a laminated structure from a light output end face along theaxis line of the semiconductor optical amplifier. In the semiconductoroptical amplifiers according to the first embodiment to the thirdembodiment of the invention, further, the carrier non-injection regionmay be also provided in an internal region of the laminated structurefrom a light incident end face along the axis line of the semiconductoroptical amplifier.

Further, in the semiconductor optical amplifiers according to the firstembodiment to the third embodiment of the invention, it is possible thatthe second electrode is not provided in the carrier non-injectionregion, or it is possible that the second electrode is composed of afirst section and a second section that are separated by an isolationtrench, and the second section of the second electrode is provided inthe carrier non-injection region. In the latter case, a voltage equal toor less than a built-in voltage is desirably applied to the secondsection of the second electrode. Specifically, a voltage equal to orless than (1.2398/λ) is desirably applied to the second section of thesecond electrode. λ represents wavelength of incident laser light to thesemiconductor optical amplifier (unit: μm), and “1.2398” representsconstant number. For example, in the case where 0.4 μm wavelength laserlight enters the semiconductor optical amplifier, a voltage equal to orless than 3.0995 volt is desirably applied. Though not limited, as thelower limit value of voltage applied to the second section of the secondelectrode, −20 volt can be exemplified. Light amplification as aninherent function of the semiconductor optical amplifier is able to beperformed by applying a voltage to the first section of the secondelectrode, while monitoring light intensity and measurement for positionadjustment and the like are able to be performed by applying a voltageto the second section of the second electrode. For such a point, adescription will be given in detail later. Further, near-field image isable to be controlled.

Further, in the semiconductor optical amplifiers according to the firstembodiment to the third embodiment of the invention, the axis line ofthe semiconductor optical amplifier may intersect with the axis line ofthe ridge stripe structure at a given angle. As given angle θ, 0.1deg≦θ≦10 deg can be exemplified. The axis line of the ridge stripestructure is a straight line that connects the point obtained by equallydividing the line between both ends of the ridge stripe structure in thelight output end face with the point obtained by equally dividing theline between both ends of the ridge stripe structure in the lightincident end face.

Further, in the semiconductor optical amplifiers according to the firstembodiment to the third embodiment of the invention, a low reflectivecoating layer formed from a laminated structure composed of at least twotypes of layers selected from the group consisting of a titanium oxidelayer, a tantalum oxide layer, a zirconia oxide layer, a silicon oxidelayer, and an aluminum oxide layer may be formed in the light incidentend face and the light output end face.

Further, in the semiconductor optical amplifiers according to the firstembodiment to the third embodiment of the invention, though not limited,the light intensity density of laser light outputted from thesemiconductor optical amplifier may be 60 kilowatt or more per 1 cm² ofa third compound semiconductor structuring the light output end face,and may be preferably 600 kilowatt or more.

Further, in the semiconductor optical amplifiers according to the firstembodiment to the third embodiment of the invention, the value of (widthof the ridge stripe structure in the light output end face)/(width oflaser light outputted from the semiconductor optical amplifier) may befrom 1.1 to 10 both inclusive, and may be preferably from 1.1 to 5 bothinclusive.

Further, in the semiconductor optical amplifiers according to the firstembodiment to the third embodiment of the invention (hereinafter, insome cases, such semiconductor optical amplifiers will be genericallyand simply referred to as “semiconductor optical amplifier of theembodiment of the invention,”) the semiconductor optical amplifier maybe composed of a transmissive semiconductor optical amplifier. However,the semiconductor optical amplifier is not limited to the transmissivesemiconductor optical amplifier. For example, the semiconductor opticalamplifier may be composed of a monolithic semiconductor opticalamplifier.

In the semiconductor optical amplifier of the embodiment of theinvention, W_(out)>W_(in) is satisfied where the width of the ridgestripe structure in the light output end face is W_(out), and the widthof the ridge stripe structure in the light incident end face is W_(in).In this case, each end section of the ridge stripe structure may becomposed of one line segment (the semiconductor optical amplifiersaccording to the first embodiment and the second embodiment of theinvention), or may be composed of two or more line segments (thesemiconductor optical amplifiers according to the first embodiment tothe third embodiment of the invention). In the former case, for example,the width of the ridge stripe structure is gently and flatly extended intapered shape from the light incident end face to the light output endface. Meanwhile, in the latter case and in the semiconductor opticalamplifiers according to the first embodiment and the second embodimentof the invention, for example, the width of the ridge stripe structureis firstly the same, and is next gently and flatly extended in taperedshape from the light incident end face to the light output end face.Further, in the latter case and in the semiconductor optical amplifieraccording to the second embodiment of the invention, for example, thewidth of the ridge stripe structure is firstly widened, and is nextnarrowed after the width exceeds the maximum width from the lightincident end face to the light output end face.

In the semiconductor optical amplifier according to the first embodimentof the invention or the semiconductor optical amplifiers according tothe second embodiment to the third embodiment of the invention, thecarrier non-injection region is provided in the internal region of thelaminated structure from the light output end face along the axis lineof the semiconductor optical amplifier. As length of the carriernon-injection region along the axis line of the semiconductor opticalamplifier (width of the carrier non-injection region) L_(NC), a valuefrom 0.1 μm to 100 μm both inclusive can be exemplified.

Further, in the semiconductor optical amplifier according to the firstembodiment of the invention or the semiconductor optical amplifiersaccording to the second embodiment to the third embodiment of theinvention, the second electrode is composed of the first section and thesecond section that are separated by the isolation trench, and thesecond section of the second electrode is provided in the carriernon-injection region. When the length of the first section is L_(Amp-1)and the length of the second section is L_(Amp-2),0.001≦L_(Amp-2)/L_(Amp-1)≦0.01 is desirably satisfied, and0.0025≦L_(Amp-2)/L_(Amp-1)≦0.01 is preferably satisfied. The electricresistance value between the first section and the second section of thesecond electrode in the semiconductor optical amplifier is 1×10²Ω ormore, is preferably 1×10³Ω or more, and is more preferably 1×10⁴Ω ormore. Further, the electric resistance value between the first sectionand the second section of the second electrode is 1×10 times or more theelectric resistance value between the second electrode and the firstelectrode, is preferably 1×10² times or more the electric resistancevalue between the second electrode and the first electrode, and is morepreferably 1×10³ times or more the electric resistance value between thesecond electrode and the first electrode. Further, the width of theisolation trench that separates the second electrode into the firstsection and the second section is desirably 1 μm or more and 50% or lessthe length of the semiconductor optical amplifier, and is preferably 10μm or more and 10% or less as much as the length of the semiconductoroptical amplifier. Further, as the width of the isolation trench, avalue from 3 μm to 20 μm both inclusive can be exemplified. As thelength of the second section of the second electrode L_(Amp-2), a valuefrom 3 μm to 100 μm both inclusive can be exemplified.

In the semiconductor optical amplifier of the embodiment of theinvention, a laser light source may be composed of a mode locking laserdiode device, and pulse laser light outputted from the mode lockinglaser diode device may enter the semiconductor optical amplifier. Inthis case, the laser light source may output pulse laser light based onmode locking operation. However, the laser light source is not limitedthereto. A known continuous oscillation laser light source, knownvarious types of pulse oscillation laser light sources such as a gainswitching laser light source and a loss switching laser light source (Qswitching laser light source), and a laser light source such as atitanium sapphire laser are able to be used. The semiconductor opticalamplifier of the embodiment of the invention is an amplifier thatdirectly amplifies an optical signal in a state of light withoutconverting the optical signal to an electric signal. The semiconductoroptical amplifier of the embodiment of the invention has a laserstructure excluding resonator effect as much as possible, and amplifiesincident light by light gain of the semiconductor optical amplifier.That is, the semiconductor optical amplifier of the embodiment of theinvention may substantially have the same composition and the sameconfiguration as those of the laser diode device structuring the laserlight source of the embodiment of the invention, and may have acomposition and a configuration different from those of the laser diodedevice structuring the laser light source of the embodiment of theinvention.

In the semiconductor optical amplifier of the embodiment of theinvention, in the case where the laser light source is composed of themode locking laser diode device as described above, the mode lockinglaser diode device (hereinafter referred to as “mode locking laser diodedevice of the embodiment of the invention”) may include: a laminatedstructure in which a first compound semiconductor layer that has a firstconductivity type and is composed of GaN compound semiconductor, a thirdcompound semiconductor layer that has a light emitting region composedof GaN compound semiconductor, and a second compound semiconductor layerthat has a second conductivity type different from the firstconductivity type and is composed of GaN compound semiconductor aresequentially layered; the second electrode formed on the second compoundsemiconductor layer; and a first electrode electrically connected to thefirst compound semiconductor layer. The laminated structure may beformed on a compound semiconductor substrate having polarity. The thirdcompound semiconductor layer may have a quantum well structure includinga well layer and a barrier layer. In addition, though not limited, thethickness of the well layer is from 1 nm to 10 nm both inclusive, and ispreferably from 1 nm to 8 nm both inclusive. The impurity dopingconcentration of the barrier layer is from 2×10¹⁸ cm⁻³ to 1×10²⁰ cm⁻³both inclusive, and is preferably from 1×10¹⁹ cm⁻³ to 1×10²⁰ cm⁻³ bothinclusive.

In driving the mode locking laser diode device of the embodiment of theinvention, light pulse may be generated in the light emitting region byflowing a current from the second electrode to the first electrodethrough the laminated structure. Further, in the mode locking laserdiode device of the embodiment of the invention, light pulse may begenerated in the light emitting region by flowing a current from thesecond electrode to the first electrode through the laminated structure.

To enable mode locking operation of the laser diode device, the lightemitting region and a saturable absorption region should be provided forthe laser diode device. Based on arrangement state of the light emittingregion and the saturable absorption region, the laser diode device isable to be generally categorized into SAL (saturable absorber layer)type or WI (weakly index guide) type in which the light emitting regionand the saturable absorption region are arranged in vertical direction,and multielectrode type including bisection type in which the lightemitting region and the saturable absorption region are arranged in linein the resonator direction. In the mode locking method, in a cubic(mainly GaAs) laser diode device, it has been confirmed that light pulsewith time width of 0.6 psec is able to be generated (see “Appl. Phys.Lett. 39 (1981) 525,” H. Yokoyama J. P. van Der Ziel, et al.). Further,in the hexagonal (mainly GaAs) laser diode device, S. Gee et al. firstlyreported generation of light pulse by using mode locking method in 2001(see “Appl. Phys. Lett. 79 (2001) 1951,” S. Gee and J. E. Bowers).However, according to “Appl. Phys. Lett. 79 (2001) 1951,” the time widthof light pulse is 30 psec, which is still long. Further, in the casewhere the multielectrode laser diode device is fabricated by using asubstrate having polarity, specifically, for example, in the case wherea multielectrode GaN laser diode device is provided on {0001} plane (Cplane) of a GaN substrate, in some cases, saturable absorption isdifficult to be controlled electrically due to QCSE effect (quantumconfinement Stark effect) by internal electric field resulting frompiezoelectric polarization or intrinsic polarization. That is, it hasbeen found that in some cases, it is necessary to increase directcurrent value flown to the first electrode for obtaining mode lockingoperation and reverse bias voltage value applied to the saturableabsorption region, subpulse component associated with main pulse isgenerated, or synchronization is difficult to be obtained between anexternal signal and light pulse.

In the mode locking laser diode device of the embodiment of theinvention, it is preferable that the thickness of the well layercomposing the third compound semiconductor layer is defined as a valuefrom 1 nm to 10 nm both inclusive, and the impurity doping concentrationof the barrier layer composing the third compound semiconductor layer isdefined as a value from 2×10¹⁸ cm⁻³ to 1×10²⁰ cm⁻³ both inclusive. Thatis, the thickness of the well layer is decreased, and carrier of thethird compound semiconductor layer is increased. In the result,influence of piezoelectric polarization is able to be decreased, and alaser light source capable of generating single-peaked light pulse thathas a short time width and small subpulse component is able to beobtained. Further, mode locking drive is able to be retained with lowreverse bias voltage, and light pulse train in synchronization with anexternal signal (electric signal and optical signal) is able to begenerated. Accordingly, the mode locking laser diode device of theembodiment of the invention is able to be applied as an oscillator thatgenerates light pulse in a volumetric optical disc system, for example.

In the mode locking laser diode device of the embodiment of theinvention, the third compound semiconductor layer may further includethe saturable absorption region, the second electrode may be separatedinto the first section for obtaining forward bias state by flowing acurrent to the first electrode through the light emitting region and thesecond section for adding electric field to the saturable absorptionregion by an isolation trench. The forward bias state may be obtained byflowing a current from the first section of the second electrode to thefirst electrode through the light emitting region, and electric fieldmay be added to the saturable absorption region by applying a voltagebetween the first electrode and the second section of the secondelectrode.

It is desirable that a reverse bias voltage is applied between the firstelectrode and the second section (that is, the first electrode is acathode and the second section is an anode). A pulse current or a pulsevoltage in synchronization with a pulse current or a pulse voltageapplied to the first section of the second electrode, or direct currentbias may be applied to the second section of the second electrode.

Further, in the mode locking laser diode device of the embodiment of theinvention, the electric resistance value between the first section andthe second section of the second electrode is 1×10²Ω or more, ispreferably 1×10³Ω or more, and is more preferably 1×10⁴Ω or more.Further, the electric resistance value between the first section and thesecond section of the second electrode is desirably 1×10 times or moreas much as the electric resistance value between the second electrodeand the first electrode, is preferably 1×10² times or more the electricresistance value between the second electrode and the first electrode,and is more preferably 1×10³ times or more the electric resistance valuebetween the second electrode and the first electrode.

In the case where the electric resistance value between the firstsection and the second section of the second electrode is 1×10²Ω ormore, or the electric resistance value between the first section and thesecond section of the second electrode is 10 times or more the electricresistance value between the second electrode and the first electrode,flow of leakage current from the first section to the second section ofthe second electrode is able to be inhibited securely. That is, acurrent injected to the light emitting region (carrier injection region,gain region) is able to be increased. At the same time, reverse biasvoltage V_(sa) applied to the saturable absorption region (carriernon-injection region) is able to be increased. In addition, such a highelectric resistance value between the first section and the secondsection of the second electrode is able to be attained only byseparating the second electrode into the first section and the secondsection by the isolation trench. That is, generation of light pulse bymode locking is able to be realized more easily.

Further, in the mode locking laser diode device of the embodiment of theinvention, the width of the isolation trench that separates the secondelectrode into the first section and the second section is desirably 1μm or more and 50% or less the resonator length, and is preferably 10 μmor more and 10% or less the resonator length. As the resonator length,though 0.3 mm can be exemplified. However, the value is not limitedthereto. In the following description, the resonator direction isregarded as Z direction and the thickness direction of the laminatedstructure is regarded as Y direction. Further, the length of thesaturable absorption region may be shorter than the length of the lightemitting region. Further, the length of the second electrode (totallength of the first section and the second section) may be shorter thanthe length of the third compound semiconductor layer. Examples ofarrangement state of the first section and the second section of thesecond electrode include the following:

(1) State in which two first sections of the second electrode and onesecond section of the second electrode are provided, an end of thesecond section is opposed to one first section with one isolation trenchin between, and the other end of the second section is opposed to theother first section with the other isolation trench in between (that is,the second electrode has a structure in which the second section issandwiched between the first sections);(2) State in which one first section of the second electrode and onesecond section of the second electrode are provided, and the firstsection of the second electrode and the second section of the secondelectrode are arranged with an isolation trench in between; and(3) State in which one first sections of the second electrode and twosecond sections of the second electrode are provided, an end of thefirst section is opposed to one second section with one isolation trenchin between, and the other end of the first section is opposed to theother second section with the other isolation trench in between.Specially, the structures (1) and (2) are desirable. Further, moregenerally, examples of arrangement state of the first section and thesecond section of the second electrode include the following:(4) State that N pieces of the first sections of the second electrodeand (N−1) pieces of the second sections of the second electrode areprovided, and the second section of the second electrode is sandwichedbetween the first sections of the second electrode; and(5) State that N pieces of the second sections of the second electrodeand (N−1) pieces of the first sections of the second electrode areprovided, and the first section of the second electrode is sandwichedbetween the second sections of the second electrode. In other words, theStates 4 and 5 are described as follows:(4′) State that N pieces of light emitting regions [carrier injectionregion, gain region] and (N−1) pieces of saturable absorption regions[carrier non-injection region] are provided, and the saturableabsorption region is sandwiched between the light emitting regions; and(5′) State that N pieces of saturable absorption regions [carriernon-injection region] and (N−1) pieces of light emitting regions[carrier injection region, gain region] are provided, and the lightemitting region is sandwiched between the saturable absorption regions.

Further, in the driving method of the mode locking laser diode device ofthe embodiment of the invention, it is possible that a current is flownfrom the second electrode to the first electrode through the lightemitting region, and an external electric signal is superimposed on thefirst electrode from the second electrode through the light emittingregion. Thereby, light pulse is able to be sync with the externalelectric signal. Alternatively, an optical signal may enter from one endface of the laminated structure. Again, thereby light pulse is able tobe sync with the optical signal.

Further, in the semiconductor optical amplifier of the embodiment of theinvention or the mode locking laser diode device of the embodiment ofthe invention, doping impurity for the barrier layer may be silicon(Si). However, doping impurity is not limited thereto. As other dopingimpurity, oxygen (O) may be adopted.

Further, the mode locking laser diode device of the embodiment of theinvention may be a laser diode device having a ridge stripe typeseparate confinement heterostructure (SCH structure). Alternatively, themode locking laser diode device of the embodiment of the invention maybe a laser diode device having an oblique ridge stripe type separateconfinement heterostructure. The height of the ridge stripe structure isdesirably from 0.1 μm to 10 μm both inclusive, and is preferably from0.2 μm to 1 μm both inclusive. However, the value thereof is not limitedthereto. Further, as the width of the ridge stripe structure, 2 μm orless is able to be exemplified, and as the lower limit of the width ofthe ridge stripe structure, for example, 0.8 μm is able to beexemplified. However, the value thereof is not limited thereto.Definition of the height of the ridge stripe structure is also able tobe applied to the semiconductor optical amplifier of the embodiment ofthe invention.

The mode locking laser diode device of the embodiment of the inventionis, for example, able to be manufactured by the following method. Thatis, the mode locking laser diode device of the embodiment of theinvention is, for example, able to be manufactured by the followingmanufacturing method including the following respective steps:

(A) step of forming the laminated structure in which the first compoundsemiconductor layer that has the first conductivity type and is composedof GaN compound semiconductor, the third compound semiconductor layerthat has the light emitting region composed of GaN compoundsemiconductor and the saturable absorption region, and the secondcompound semiconductor layer that has the second conductivity typedifferent from the first conductivity type and is composed of GaNcompound semiconductor are sequentially layered is formed;(B) subsequent step of forming the strip-shaped second electrode on thesecond compound semiconductor layer;(C) subsequent step of etching at least part of the second compoundsemiconductor layer with the use of the second electrode as an etchingmask, and thereby forming the ridge stripe structure; and(D) subsequent step of forming a resist layer for forming the isolationtrench in the second electrode, and subsequently forming the isolationtrench in the second electrode by wet etching method with the use of theresist layer as a wet etching mask, and thereby separating the secondelectrode into the first section and the second section by the isolationtrench.

The ridge stripe structure is formed by adopting the foregoingmanufacturing method, that is, by etching at least part of the secondcompound semiconductor layer with the use of the strip-shaped secondelectrode as an etching mask. That is, the ridge stripe structure isformed by self alignment method by using the patterned second electrodeas an etching mask. Thus, there is no joint misalignment between thesecond electrode and the ridge stripe structure. Further, the isolationtrench is formed in the second electrode by wet etching method. Byadopting wet etching method as described above, deterioration of theoptical and electric characteristics of the second compoundsemiconductor layer is able to be inhibited differently from dry etchingmethod. Accordingly, deterioration of light emitting characteristics isable to be securely prevented.

In the step (C), part of the second compound semiconductor layer may beetched in the thickness direction, all of the second compoundsemiconductor layer may be etched in the thickness direction, the secondcompound semiconductor layer and the third compound semiconductor layermay be etched in the thickness direction, or part of the second compoundsemiconductor layer, the third compound semiconductor layer, and thefirst compound semiconductor layer may be etched in the thicknessdirection.

Further, when etching rate of the second electrode is ER₁ and etchingrate of the laminated structure is ER₁ in forming the isolation trenchin the second electrode in the foregoing step D, ER₀/ER₁≧1×10 isdesirably satisfied, and ERr₀/ER₁≧1×10² is preferably satisfied. In thecase where ER₀/ER₁ satisfies the foregoing relation, the secondelectrode is able to be securely etched without etching the laminatedstructure (or if the laminated structure is etched, the etching portionis little.)

The semiconductor optical amplifier of the embodiment of the inventionis able to be substantially manufactured by a manufacturing methodsimilar to the manufacturing method of the foregoing mode locking laserdiode device of the embodiment of the invention though depending on theform thereof. However, the manufacturing method thereof is not limitedthereto.

In the mode locking laser diode device of the embodiment of theinvention, the second electrode may be composed of a palladium (Pd)single layer, a nickel (Ni) single layer, a platinum (Pt) single layer,or a laminated structure of a palladium layer and a platinum layer inwhich the palladium layer is in contact with the second compoundsemiconductor layer, or a laminated structure of a palladium layer and anickel layer in which the palladium layer is in contact with the secondcompound semiconductor layer. In the case where the lower metal layer iscomposed of palladium, and the upper metal layer is composed of nickel,the thickness of the upper metal layer is desirably 0.1 μm or more, andis preferably 0.2 μm or more. Further, the second electrode ispreferably composed of the palladium (Pd) single layer. In this case,the thickness thereof is desirably 20 nm or more, and is preferably 50nm or more. Further, the second electrode is preferably composed of apalladium (Pd) single layer, a nickel (Ni) single layer, a platinum (Pt)single layer, or a laminated structure of a lower metal layer and anupper metal layer in which the lower metal layer is in contact with thesecond compound semiconductor layer (however, the lower metal layer iscomposed of one metal selected from the group consisting of palladium,nickel, and platinum; and the upper metal layer is composed of a metalhaving etching rate in forming the isolation trench in the secondelectrode in the foregoing step (D) equal to, similar to, or higher thanetching rate of the lower metal layer). Further, an etching liquid usedin forming the isolation trench in the second electrode in the foregoingstep (D) is desirably aqua regia, nitric acid, vitriolic acid, muriaticacid, or a mixed liquid composed of at least two types out of theseacids (specifically, a mixed liquid composed of nitric acid andvitriolic acid, or a mixed liquid composed of nitric acid and muriaticacid). The width of the second electrode is desirably from 0.5 μm to 50μm both inclusive, and is preferably from 1 μm to 5 μm both inclusive.The structure of the second electrode in the foregoing mode lockinglaser diode device of the embodiment of the invention is able to be alsoapplied to the semiconductor optical amplifier of the embodiment of theinvention though depending on the form thereof.

In the semiconductor optical amplifier of the embodiment of theinvention or the mode locking laser diode device of the embodiment ofthe invention, the laminated structure may be composed of AlGaInNcompound semiconductor. Specific examples of AlGaInN compoundsemiconductor include GaN, AlGaN, GaInN, and AlGaInN. Further, such acompound semiconductor may include boron (B) atom, thallium (Tl) atom,arsenic (As) atom, phosphorus (P) atom, or antimony (Sb) atom accordingto needs. Further, the third compound semiconductor layer structuring alight amplification region or the light emitting region (gain region)and the saturable absorption region (in some cases, the third compoundsemiconductor layer is referred to as “active layer”) has the quantumwell structure. Specifically, the third compound semiconductor layer mayhave single quantum well structure [QW structure], or multiquantum wellstructure [MQW structure]. The third compound semiconductor layer havingthe quantum well structure has a structure in which at least one welllayer and at least one barrier layer are layered. As a combination ofcompound semiconductor composing the well layer and compoundsemiconductor composing the barrier layer, (In_(y)Ga_((1-y))N, GaN),(In_(y)Ga_((1-y))N, In_(z)Ga_((1-z))N) (y>z), and (In_(y)Ga_((1-y))N,AlGaN) can be exemplified.

Further, in the mode locking laser diode device of the embodiment of theinvention, the second compound semiconductor layer may have asuperlattice structure in which a p-type GaN layer and a p-type AlGaNlayer are alternately layered. The thickness of the superlatticestructure may be 0.7 μm or less. By adopting such a superlatticestructure, while a refractive index necessary as a cladding layer ismaintained, a series resistance component of the laser diode device isable to be decreased, leading to realizing a low operation voltage ofthe laser diode device. Though the lower limit value of the thickness ofthe superlattice structure is not limited, the lower limit value is, forexample, 0.3 μm. As the thickness of the p-type GaN layer composing thesuperlattice structure, a thickness from 1 nm to 5 nm both inclusive canbe exemplified. As the thickness of the p-type AlGaN layer composing thesuperlattice structure, a thickness from 1 nm to 5 nm both inclusive canbe exemplified. As the total number of layers of the p-type GaN layerand the p-type AlGaN layer, the number from 60 layers to 300 layers bothinclusive can be exemplified. Further, the distance from the thirdcompound semiconductor layer to the second electrode may be 1 μm orless, and preferably 0.6 μm or less. By defining the distance from thethird compound semiconductor layer to the second electrode, thethickness of the p-type second compound semiconductor layer having highresistance is able to be decreased, and the operation voltage of thelaser diode device is able to be decreased. Though the lower limit valueof the distance from the third compound semiconductor layer to thesecond electrode is not limited, for example, the lower limit value ofthe distance from the third compound semiconductor layer to the secondelectrode is 0.3 μm. Further, the second compound semiconductor layermay be doped with Mg at the level of 1×10¹⁹ cm⁻³ or more. The absorptioncoefficient of the second compound semiconductor layer to light in 405nm wavelength from third compound semiconductor layer may be at least 50cm⁻¹. The atom concentration of Mg comes from material property that themaximum hole concentration is shown at the value of 2×10¹⁹ cm⁻³, and isa result of design that the maximum hole concentration, that is, thespecific resistance of the second compound semiconductor layer becomesthe minimum. The absorption coefficient of the second compoundsemiconductor layer is defined in viewpoint of decreasing resistance ofthe laser diode device as much as possible. In the result, in general,the absorption coefficient of light of the third compound semiconductorlayer becomes 50 cm⁻¹. However, it is possible that the Mg dope amountis intentionally set to the concentration of 2×10¹⁹ cm⁻³ or more inorder to increase the absorption coefficient. In this case, the upperlimit Mg dope amount for obtaining a practical hole concentration is,for example, 8×10¹⁹ cm⁻³. Further, the second compound semiconductorlayer may have a non-doped compound semiconductor layer and a p-typecompound semiconductor layer from the third compound semiconductor layerside. The distance from the third compound semiconductor layer to thep-type compound semiconductor layer may be 1.2×10⁻⁷ m or less. Bydefining the distance from the third compound semiconductor layer to thep-type compound semiconductor layer as above, internal loss is able tobe suppressed in a range in which the internal quantum efficiency is notlowered. Thereby, threshold current I_(th) at which laser oscillation isstarted is able to be decreased. Though the lower limit value of thedistance from the third compound semiconductor layer to the p-typecompound semiconductor layer is not limited, for example, the lowerlimit value is 5×10⁻⁸ m. Further, on both side faces of the ridgesection, a laminated insulating film composed of SiO₂/Si laminatedstructure may be formed. The difference between the effective refractiveindex of the ridge section and the effective refractive index of thelaminated insulating film may be from 5×10⁻³ to 1×10⁻² both inclusive.By using such a laminated insulating film, even in the case of highoutput operation exceeding 100 mW, single fundamental lateral mode isable to be maintained. Further, the second compound semiconductor layermay have a structure in which a non-doped GaInN layer (p-side lightguide layer), a non-doped AlGaN layer (p-side cladding layer), a Mgdoped AlGaN layer (electron barrier layer), a superlattice structure(superlattice cladding layer) composed of a GaN layer (Mg doped)/AlGaNlayer, and a Mg doped GaN layer (p-side contact layer) are layered fromthe third compound semiconductor layer side. The bandgap of compoundsemiconductor composing the well layer in the third compoundsemiconductor layer is desirably 2.4 eV or more. Further, the wavelengthof laser light outputted from the third compound semiconductor layer isdesirably from 360 nm to 500 nm both inclusive, and is preferably from400 nm to 410 nm both inclusive. It is needless to say that theforegoing various structures are able to be combined as appropriate. Theabove-mentioned structure in the foregoing mode locking laser diodedevice of the embodiment of the invention is also applicable to thesemiconductor optical amplifier of the embodiment of the inventionsubstantially though depending on the form thereof.

As described above, in the second compound semiconductor layer, anon-doped compound semiconductor layer (for example, a non-doped GaInNlayer or a non-doped AlGaN layer) may be formed between the thirdcompound semiconductor layer and the electron barrier layer. Further, anon-doped GaInN layer as a light guide layer may be formed between thethird compound semiconductor layer and the non-doped compoundsemiconductor layer. The uppermost layer of the second compoundsemiconductor layer may have a structure occupied by a Mg doped GaNlayer (p-side contact layer).

Various GaN compound semiconductor layers composing the semiconductoroptical amplifier of the embodiment of the invention or the mode lockinglaser diode device of the embodiment of the invention are sequentiallyformed over a substrate. Examples of the substrate include a GaAssubstrate, a GaN substrate, an SiC substrate, an alumina substrate, aZnS substrate, a ZnO substrate, an MN substrate, a LiMgO substrate, aLiGaO₂ substrate, a MgAl₂O₄ substrate, an InP substrate, an Sisubstrate, and a laminated body in which a foundation layer and a bufferlayer are formed on the surface (main face) of the foregoing substratein addition to a sapphire substrate. Mainly, in the case where the GaNcompound semiconductor layer is formed on the substrate, the GaNsubstrate has the preference due to its small defect density. However,it is known that in the GaN substrate, its characteristics are changedfrom/to polarity, non-polarity, and semi-polarity according to thegrowth plane. Further, examples of methods of forming the various GaNcompound semiconductor layers composing the semiconductor opticalamplifier of the embodiment of the invention or the mode locking laserdiode device of the embodiment of the invention include metal organicchemical vapor deposition method (MOCVD method and MOVPE method),molecular beam epitaxy method (MBE method), and hydride vapor growthmethod in which halogen contributes to transfer or reaction and thelike.

Examples of organic gallium source gas in MOCVD method include trimethylgallium (TMG) gas and triethyl gallium (TEG) gas. Examples of nitrogensource gas include ammonia gas and hydrazine gas. In forming the GaNcompound semiconductor layer having n-type conductivity type, forexample, silicon (Si) may be added as n-type impurity (n-type dopant).In forming the GaN compound semiconductor layer having p-typeconductivity type, for example, magnesium (Mg) may be added as p-typeimpurity (p-type dopant). Further, in the case where aluminum (Al) orindium (In) is contained as a component atom of the GaN compoundsemiconductor layer, trimethyl aluminum (TMA) gas may be used as an Alsource, and trimethyl indium (TMI) gas may be used as an In source.Further, monosilane gas (SiH₄ gas) may be used as a Si source, andciclopentadienyl magnesium gas, methylciclopentadienyl magnesium, orbisciclopentadienyl magnesium (Cp₂Mg) may be used as a Mg source.Examples of n-type impurity (n-type dopant) include Ge, Se, Sn, C, Te,SO, Pd, and Po in addition to Si. Examples of p-type impurity (p-typedopant) include Zn, Cd, Be, Ca, Ba, C, Hg, and Sr in addition to Mg.

When the first conductive type is n type, the first electrodeelectrically connected to the first compound semiconductor layer havingn-type conductivity type desirably has a single layer structure or amultilayer structure containing at least one metal selected from thegroup consisting of gold (Au), silver (Ag), palladium (Pd), Al(aluminum), Ti (titanium), tungsten (W), Cu (copper), Zn (zinc), tin(Sn) and indium (In), and for example, Ti/Au, Ti/Al, and Ti/Pt/Au areable to be exemplified. The first electrode is electrically connected tothe first compound semiconductor layer. The first electrode may beformed on the first compound semiconductor layer, and the firstelectrode may be connected to the first compound semiconductor layerwith a conductive material layer or a conducive substrate in between.The first electrode and the second electrode are able to be formed byPVD method such as vacuum evaporation method and sputtering method.

A pad electrode may be provided on the first electrode and the secondelectrode in order to obtain electrical connection to an externalelectrode or a circuit. The pad electrode desirably has a single layerstructure or a multilayer structure containing at least one metalselected from the group consisting of Ti (titanium), aluminum (Al), Pt(platinum), Au (gold), and Ni (nickel). Otherwise, the pad electrode mayhave a multilayer structure exemplified as a Ti/Pt/Au multilayerstructure and a Ti/Au multilayer structure.

The mode locking laser diode device of the embodiment of the inventionmay further include an external reflecting mirror. That is, the modelocking laser diode device of the embodiment of the invention may be anexternal resonator type mode locking laser diode device. Alternatively,the mode locking laser diode device of the embodiment of the inventionmay be a monolithic mode locking laser diode device. The externalresonator type mode locking laser diode device may be light condensingtype, or collimation type. In the external resonator type mode lockinglaser diode device, light reflectance on one end face of a laminatedstructure from which light pulse is outputted is preferably 0.5% orless. The light reflectance value is significantly lower than the lightreflectance on one end face of a laminated structure from which lightpulse is outputted in existing laser diode devices (in general, from 5%to 10% both inclusive). In the external resonator mode locking laserdiode device, the value of the external resonator (Z′, unit mm)desirably satisfies 0<Z′<1500, and is preferably satisfies 30≦Z′≦150.

The embodiment of the invention is applicable to various fields such asthe optical disc system, the communication field, the opticalinformation field, the photoelectronic integration circuit, the fieldapplying nonlinear optical phenomenon, the optical switch, the lasermeasurement field and various analysis fields, the ultrafastspectroscopy field, the multiphoton excitation spectroscopy field, themass analysis field, the microspectroscopic field using multiphotonabsorption, quantum control of chemical reaction, the nanothree-dimensional processing field, various processing fields applyingmultiphoton absorption, the medical field, and the bio imaging field.

First Example

The first example relates to the semiconductor optical amplifieraccording to the first embodiment of the invention. FIG. 1 illustrates aconceptual view of a light output device of the first example includingthe semiconductor optical amplifier. FIG. 2 illustrates a schematiccross sectional view of the semiconductor optical amplifier where thesemiconductor optical amplifier is cut along a virtual vertical plane(YZ plane) including an axis line of the semiconductor optical amplifier(direction in which a light guide extends, and is referred to as “Zdirection” for convenience sake). FIG. 3 illustrates a schematic crosssectional view of the semiconductor optical amplifier where thesemiconductor optical amplifier is cut along a virtual vertical plane(XY plane) orthogonal to the axis line of the semiconductor opticalamplifier. FIG. 2 is a schematic cross sectional view taken along lineI-I of FIG. 3. FIG. 3 is a schematic cross sectional view taken alongline II-II of FIG. 2. FIG. 4 illustrates a schematic perspective view ofthe semiconductor optical amplifier. FIG. 5 illustrates a schematic planview of a ridge stripe structure.

The light output device of the first example includes a laser lightsource 100 and a semiconductor optical amplifier 200 that opticallyamplifies laser light from the laser light source 100 and outputsamplified light.

As illustrated in FIG. 1, the semiconductor optical amplifier 200 iscomposed of a transmissive semiconductor optical amplifier. Lowreflective coating layers (AR) 202 and 204 are formed on a lightincident end face 201 of the semiconductor optical amplifier 200 and alight output end face 203 opposed to the light incident end face 201.The low reflecting coating layers 202 and 204 have a structure in whicha titanium oxide layer and an aluminum oxide layer are layered. Thelaser light entering from the light incident end face 201 side isoptically amplified inside the semiconductor optical amplifier 200, andis outputted from the light output end face 203 on the opposite side ofthe light incident end face 201 side. The laser light is fundamentallyguided in only one direction. Further, in the first example, the laserlight source 100 is composed of a known continuous oscillation laserequipment. Laser light outputted from the laser equipment enters thesemiconductor optical amplifier 200.

In the light output device of the first example illustrated in FIG. 1,laser light outputted from the laser light source 100 enters areflecting mirror 20 through a light isolator 15 and a reflecting mirror16. Laser light reflected by the reflecting mirror 20 passes through ahalf-wave plate (λ/2 wave plate) 21 and a lens 22, and enters thesemiconductor optical amplifier 200. The light isolator 15 is arrangedto prevent returned light from the semiconductor optical amplifier 200from heading for the laser light source 100. The laser light isoptically amplified in the semiconductor optical amplifier 200, and isoutputted outside the system through a lens 30.

The semiconductor optical amplifier 200 includes: a laminated structurein which a first compound semiconductor layer 230 that has a firstconductivity type (in the first example, specifically, n-typeconductivity type) and is composed of GaN compound semiconductor, athird compound semiconductor layer (active layer) 240 that has a lightamplification region (carrier injection region, gain region) 241composed of GaN compound semiconductor, and a second compoundsemiconductor layer 250 that has a second conductivity type differentfrom the first conductivity type (in the first example, specifically,p-type conductivity type) and is composed of GaN compound semiconductorare sequentially layered; a second electrode 262 formed on the secondcompound semiconductor layer 250; and a first electrode 261 electricallyconnected to the first compound semiconductor layer 230.

In the semiconductor optical amplifier 200 of the first example, thelaminated structure has the ridge stripe structure. When the width ofthe ridge stripe structure in the light output end face 203 is W_(out),and the width of the ridge stripe structure in the light incident endface 201 is W_(in), W_(out)>W_(in) is satisfied. Specifically, W_(out)is 15 82 m and W_(in) is 1.4 μm. A carrier non-injection region 205 isprovided in an internal region of the laminated structure from the lightoutput end face 203 along axis line AX₁ of the semiconductor opticalamplifier 200. When length of the carrier non-injection region 205 alongthe axis line AX₁ of the semiconductor optical amplifier 200 (width ofthe carrier non-injection region 205) is L_(NC), L_(NC)=5 μm issatisfied. The second electrode 262 is not provided in the carriernon-injection region 205. The length of the entire semiconductor opticalamplifier is 2.0 mm. The carrier non-injection region is also providedin an internal region of the laminated structure from the light incidentend face 201 along the axis line of the semiconductor optical amplifier200.

More specifically, the semiconductor optical amplifier 200 of the firstexample has a ridge stripe type separate confinement heterostructure(SCH structure). In addition, the semiconductor optical amplifier 200 ofthe first example has a structure similar to a GaN laser diode structurecomposed of an index guide type AlGaInN. The width of the ridge stripestructure is gently and flatly extended in tapered shape from the lightincident end face 201 to the light output end face 203. Further, theaxis line AX₁ of the semiconductor optical amplifier 200 intersects withaxis line AX₂ of the ridge stripe structure at a given angle,specifically at θ=5.0 deg. The axis line AX₁ and the axis line AX₂ areindicated by dotted lines in FIG. 5.

The laminated structure is formed on a compound semiconductor substrate221. Specifically, the semiconductor optical amplifier 200 is providedon (0001) plane of the n-type GaN substrate 221. The (0001) plane of then-type GaN substrate 221 is also called “C plane,” and is a crystalplane having polarity. The first compound semiconductor layer 230, thethird compound semiconductor layer 240, and the second compoundsemiconductor layer 250 are specifically composed of AlGaInN compoundsemiconductor. More specifically, the first compound semiconductor layer230, the third compound semiconductor layer 240, and the second compoundsemiconductor layer 250 have a layer structure illustrated in thefollowing Table 1. In Table 1, the listed items are shown in the orderfrom the layer farthest from the n-type GaN substrate 221 to the layerclosest to the n-type GaN substrate 221. The bandgap of compoundsemiconductor composing the well layer in the third compoundsemiconductor layer 240 is 3.06 eV. The third compound semiconductorlayer 240 has a quantum well structure including a well layer and abarrier layer. The doping concentration of impurity (specifically,silicon (Si)) of the barrier layer is from 2×10¹⁷ cm⁻³ to 1×10²⁰ cm⁻³both inclusive.

TABLE 1 Second compound semiconductor layer 250 p-type GaN contact layer(Mg doped) 257 p-type AlGaN (Mg doped) cladding layer 255 p-type GaN (Mgdoped) layer 254 p-type AlGaN electron barrier layer (Mg doped) 253Third compound semiconductor layer 240 GaInN quantum well active layer(well layer: Ga_(0.92) In_(0.08) N/barrier layer: Ga_(0.95)In_(0.02)N)First compound semiconductor layer 230 n-type GaN layer 232 n-type AlGaNcladding layer 231 well layer (two layers): 10 nm [non-doped] barrierlayer (three layers): 12 nm [doping concentration (Si): 2 × 10¹⁵ cm⁻³]

Further, part of the p-type GaN contact layer 257 and part of the p-typeAlGaN cladding layer 255 are removed by RIE method, and a ridge stripestructure (ridge section 258) is formed. On both sides of the ridgesection 258, a laminated insulating film 259 composed of SiO₂/Si isformed. The SiO₂ layer is the lower layer and the Si layer is the upperlayer. The difference between the effective refractive index of theridge section 258 and the effective refractive index of the laminatedinsulating film 259 is from 5×10⁻³ to 1×10⁻² both inclusive, andspecifically 7×10⁻³. On the p-type GaN contact layer 257 correspondingto the top face of the ridge section 258 to part of the top face of thep-type AlGaN cladding layer 255, the second electrode (p-type ohmicelectrode) 262 is formed. Meanwhile, on the rear face of the n-type GaNsubstrate 221, the first electrode (n-type ohmic electrode) 261 composedof Ti/Pt/Au is formed. In the first example, the second electrode 262 isformed from a Pd single layer having a thickness of 0.1 μm.

The thickness of the p-type AlGaN cladding layer 255 is 400 nm. Thethickness of the p-type GaN layer 254 is 100 nm. The thickness of thep-type GaN electron barrier layer 253 is 20 nm. The thickness of thep-type GaN contact layer 257 is 100 nm. Further, the p-type AlGaNelectron barrier layer 253, the p-type GaN layer 254, and the p-typeAlGaN cladding layer 255, and the p-type GaN contact layer 257 composingthe second compound semiconductor layer 250 are doped with Mg at thelevel of 1×10¹⁹ cm⁻³ or more (specifically at the level of 2×10¹⁹ cm⁻³).Meanwhile, the thickness of the n-type AlGaN cladding layer 231 is 2.5μm, and the thickness of the n-type GaN layer 232 is 200 nm. When thethickness of the n-type compound semiconductor layer sandwiched betweenthe n-type AlGaN cladding layer 231 and the third compound semiconductorlayer 240 (thickness of the n-type GaN layer 232) is t₁, and thethickness of the p-type compound semiconductor layer sandwiched betweenthe p-type AlGaN cladding layer 255 and the third compound semiconductorlayer 240 (total thickness of the p-type GaN layer 254 and the p-typeAlGaN electron barrier layer 253) is t₂, t₁=200 nm, t₂=120 nm, and0.1≦t₂/t₁<1 are satisfied.

In the first example, single mode continuous oscillation laser light inwavelength of 405 nm (light output: 15 milliwatt) entered thesemiconductor optical amplifier 200. A direct current of 600 milliamperewas flown from the second electrode 262 to the first electrode 261. Inthe case where the value of the direct current is converted to a valueper 1 cm² of the third compound semiconductor layer 240 structuring thelight incident end face 201, the value is 3.7×10³ ampere/cm². Anear-field image of laser light outputted from the semiconductor opticalamplifier 200 at this time is illustrated in FIG. 6A. As Comparativeexample 1, a semiconductor optical amplifier having the same compositionand the same structure as those of first example was fabricated, exceptthat the carrier non-injection region 205 was not provided. A near-fieldimage of laser light outputted from a semiconductor optical amplifier ofComparative example 1 in the case where a direct current of 600milliampere was flown from the second electrode to the first electrodeis illustrated in FIG. 6B. As evidenced by FIG. 6B, in the semiconductoroptical amplifier of Comparative example 1, the width of the near-fieldimage was narrower than W_(out) (15 μm), and its 1/e² width was 5 μm(half bandwidth: 3.1 μm). The inventors of the invention firstly foundthat such a phenomenon is a phenomenon unique to nitride semiconductorsystem semiconductor optical amplifiers. Such a narrow near-field imageadversely affects saturation and reliability of amplified light output.The light intensity density of laser light outputted from thesemiconductor optical amplifier of Comparative example 1 was 47milliwatt. Meanwhile, as evidenced by FIG. 6A, in the semiconductoroptical amplifier 200 of the first example, the mode field wasbroadened, the width of the near-field image was wide and its 1/e² widthwas 11.5 μm (half bandwidth: 5.8 μm). Further, the light intensitydensity of laser light outputted from the semiconductor opticalamplifier 200 of the first example was 122 milliwatt. The amplifiedlight output was higher than that of the semiconductor optical amplifierof Comparative example 1. As described above, it was confirmed that theamplified light output was significantly increased by providing thecarrier non-injection region 205. Further, the value of (width of theridge stripe structure in the light output end face)/(width of the laserlight outputted from the semiconductor optical amplifier) was 1.3. Thewidth of the laser light outputted from the semiconductor opticalamplifier means 1/e² width in a near-field image, that is, width of anear-field image having 1/e² intensity to the peak intensity.

It is presumed that the width of the near-field image of outputted laserlight is broadened by providing the carrier non-injection region 205 forthe following reason. That is, carrier distribution in the laminatedstructure in XY plane is in the shape of a mountain having one peak ifthe light intensity of the incident laser light is low. However, if thelight intensity of the incident laser light is increased, carrierinjection/diffusion in the laminated structure of the semiconductoroptical amplifier are not performed sufficiently, and thus the carrierdistribution in the laminated structure in XY plane becomes in the shapeof a mountain having two peaks (mountain/valley/mountain-like lightintensity pattern). It is known that in the case where the number ofcarriers is decreased in the compound semiconductor layer, relativerefractive index of the compound semiconductor layer is increased. Thus,laser light outputted from the light output end face of thesemiconductor optical amplifier is difficult to be broadened in thewidth direction, and the width of the near-field image was narrower thanW_(out). In addition, since the area occupying the light output end faceof the region from which laser light is outputted from the semiconductoroptical amplifier is small, high output of the semiconductor opticalamplifier is difficult to be realized.

Meanwhile, in the semiconductor optical amplifier of the first example,the carrier non-injection region 205 not contributing to lightamplification is provided. Thus, even if the light intensity of incidentlaser light is increased, the phenomenon that relative refractive indexis increased due to carrier distribution is able to be suppressed frombeing generated. Thus, laser light outputted from the light output endface of the semiconductor optical amplifier is easily broadened in thewidth direction. In addition, since the area occupying the light outputend face of the region from which laser light is outputted from thesemiconductor optical amplifier is large, high output of thesemiconductor optical amplifier is able to be realized.

Second Example

The second example is a modification of the first example. FIG. 7illustrates a conceptual view of a light output device of the secondexample including the semiconductor optical amplifier. FIG. 8illustrates a schematic cross sectional view of the semiconductoroptical amplifier where the semiconductor optical amplifier is cut alonga virtual vertical plane (YZ plane) including an axis line (Z direction)of the semiconductor optical amplifier. FIG. 9 illustrates a schematiccross sectional view of the semiconductor optical amplifier where thesemiconductor optical amplifier is cut along a virtual vertical plane(XY plane) orthogonal to the axis line of the semiconductor opticalamplifier. FIG. 8 is a schematic cross sectional view taken along lineI-I of FIG. 9. FIG. 9 is a schematic cross sectional view taken alongline II-II of FIG. 8. Further, FIG. 11 illustrates a schematicperspective view of the semiconductor optical amplifier, and FIG. 12illustrates a schematic plan view of a ridge stripe structure.

Further, FIG. 10 illustrates a schematic end view along the direction inwhich a resonator of a mode locking laser diode device configuring alaser light source extends (a schematic end view where the mode lockinglaser diode device is cut along the YZ plane). A schematic crosssectional view taken along the direction perpendicular to the directionin which the resonator extends (schematic cross sectional view where themode locking laser diode device is cut along the XY plane) is the sameas FIG. 9 except for referential numbers. That is, the 200s referentialnumbers in FIG. 9 correspond to 100s referential numbers of theschematic cross sectional view taken along the direction perpendicularto the direction in which the resonator extends. FIG. 10 is a schematicend view similar to the view taken along line I-I of FIG. 9. Further,FIG. 19A schematically illustrates a system of performing mode lockingdrive by configuring an external resonator from the mode locking laserdiode device of the second example.

In the second example, a second electrode 262 is composed of a firstsection 262A and a second section 262B that are separated by anisolation trench 262C, and the second section 262B of the secondelectrode is provided in a carrier non-injection region 205. A voltageequal to or less than a built-in voltage is applied to the secondsection 262B of the second electrode. Specifically, 0 volt is applied tothe second section 262B of the second electrode. Light amplification asan inherent function of the semiconductor optical amplifier 200 is ableto be performed by applying a voltage to the first section 262A of thesecond electrode, while measurement for position adjustment and the likeis able to be performed by applying a voltage to the second section 262Bof the second electrode.

In the second example, when the length of the first section 262A isL_(Amp-1), and the length of the second section 262B is L_(Amp-2),L_(Amp-1)=1.97 mm, L_(Amp-2)=0.01 mm, and 0.001≦L_(Amp-2)/L_(Amp-1)≦0.01are satisfied. Further, the width of the isolation trench is 0.02 mm.

In the second example, a voltage applied to the second section of thesecond electrode is lower the voltage applied to the first section ofthe second electrode. Thereby, since the carrier non-injection regionincluding the second section exists, even if the light intensity ofincident laser light is increased, the phenomenon that relativerefractive index of the compound semiconductor layer is increased isable to be inhibited from being generated. In addition, laser lightoutputted from the light output end face of the semiconductor opticalamplifier is easily broadened in the width direction. Further, since thearea occupying the light output end face of the region from which laserlight is outputted from the semiconductor optical amplifier is large,high output of the semiconductor optical amplifier is able to berealized.

The light output device of the second example includes: the laser lightsource 100; the semiconductor optical amplifier 200 that opticallyamplifies laser light from the laser light source 100 and outputsamplified light; an alignment device 300 that adjusts relative positionof the semiconductor optical amplifier with respect to laser lightentering the semiconductor optical amplifier 200; and a semiconductoroptical amplifier control device 400 that controls operation of thesemiconductor optical amplifier 200.

The semiconductor optical amplifier control device 400 is specificallycomposed of a combination of a known direct current electric powersource, a voltage measurement device, and a current measurement device.Further, resolution capability of a voltage monitor in the semiconductoroptical amplifier control device 400 is 1 millivolt or less, and is morespecifically 0.1 millivolt or less. Further, resolution capability of acurrent monitor in the semiconductor optical amplifier control device400 is 100 microampere or less, and is more specifically 10 microampereor less.

In the second example, the laser light source 100 is composed of a modelocking laser diode device, and pulse laser light outputted from themode locking laser diode device enters the semiconductor opticalamplifier 200. In this case, the laser light source 100 outputs pulselaser light based on mode locking operation. For details of the modelocking laser diode device, a description will be given later. Thesemiconductor optical amplifier 200 in the second example substantiallyhas the same composition and the same configuration as those of a modelocking laser diode device 110 configuring the laser light source 100 inthe second example, except for the composition and the structure of thesecond electrode.

In the light output device of the second example illustrated in FIG. 7,the laser light source 100 is composed of the mode locking laser diodedevice 110, a lens 11, an optical filter 12, an external mirror 13, anda lens 14. Laser light outputted from the laser light source 100 entersa reflecting mirror 20 through the light isolator 15 and the reflectingmirror 16. Laser light reflected by the reflecting mirror 20 passesthrough the half-wave plate (λ/2 wave plate) 21 and the lens 22, andenters the semiconductor optical amplifier 200. Light is amplified inthe semiconductor optical amplifier 200, and is outputted outside thesystem through the lens 30. The reflecting mirror 20, the half-waveplate 21, and the lens 22 are laid on the alignment device 300. Thealignment device 300 is specifically composed of XYZ stage. When thethickness direction of the laminated structure in the semiconductoroptical amplifier 200 described later is Y direction and the axis linedirection of the semiconductor optical amplifier 200 is Z direction, thereflecting mirror 20 and the lens 22 are moved in the X direction, the Ydirection, and the Z direction by the alignment device 300.

The mode locking laser diode device 110 of the second example thatconfigures the laser light source 100 and has light emitting wavelengthband of 405 nm includes: a laminated structure in which a first compoundsemiconductor layer 130 that has a first conductivity type (in thesecond example, specifically, n-type conductivity type) and is composedof GaN compound semiconductor, a third compound semiconductor layer(active layer) 140 that has a light emitting region (gain region) 141composed of GaN compound semiconductor, and a second compoundsemiconductor layer 150 that has a second conductivity type differentfrom the first conductivity type (in the second example, specifically,p-type conductivity type) and is composed of GaN compound semiconductorare sequentially layered; b. a strip-shaped second electrode 162 formedon the second compound semiconductor layer 150; and a first electrode161 electrically connected to the first compound semiconductor layer130.

The laminated structure is formed on compound semiconductor substrates121 and 221 having polarity. The third compound semiconductor layers 140and 240 have a quantum well structure including a well layer and abarrier layer. The thickness of the well layer is from 1 nm to 10 nmboth inclusive. The doping concentration of impurity (specifically,silicon (Si)) of the barrier layer is from 2×10¹⁸ cm⁻³ to 1×10²⁰ cm⁻³both inclusive.

Specifically, the mode locking laser diode device 110 and thesemiconductor optical amplifier 200 of the second example have a ridgestripe type separate confinement heterostructure (SCH structure). Morespecifically, the mode locking laser diode device 110 and thesemiconductor optical amplifier 200 of the second example have a GaNlaser diode structure composed of an index guide type AlGaInN, and astraight line-like ridge section (ridge stripe structure). In addition,the mode locking laser diode device 110 and the semiconductor opticalamplifier 200 are provided on (0001) plane of an n-type GaN substrate121 and the n-type GaN substrate 221. The third compound semiconductorlayers 140 and 240 have a quantum well structure. The (0001) plane ofthe n-type GaN substrates 121 and 221 is also called “C plane,” and is acrystal plane having polarity. The first compound semiconductor layers130 and 230, the third compound semiconductor layers 140 and 240, andthe second compound semiconductor layers 150 and 250 are specificallycomposed of AlGaInN compound semiconductor. More specifically, the firstcompound semiconductor layers 130 and 230, the third compoundsemiconductor layers 140 and 240, and the second compound semiconductorlayers 150 and 250 have a layer structure illustrated in the followingTable 2. In Table 2, the listed items are shown in the order from thelayer farthest from the n-type GaN substrates 121 and 221 to the layerclosest to the n-type GaN substrates 121 and 221.

TABLE 2 Second compound semiconductor layers 150 and 250 p-type GaNcontact layer (Mg doped) 157 and 257 p-type GaN (Mg doped)/AlGaNsuperlattice cladding layers 156 and 256 p-typeAlGaN electron barrierlayer (Mg doped) 153 and 253 Non-doped AIGaN cladding layer 152 and 252Non-doped GaInN light guide layers 151 and 251 Third compoundsemiconductor layers 140 and 240 GaInN quantum well active layer (welllayer: Ga_(0.92) In_(0.08)N/barrier layer: Ga_(0.98) In_(0.02)N) Firstcompound semiconductor layers 130 and 230 n-type GaN layers 132 and 232n-type AlGaN cladding layers 131 and 231 well layer (two layers): 8 nm[non-doped] barrier layer (three layers): 10 nm [doping concentration(Si): 2 × 10¹⁸ cm⁻³]

Further, part of the p-type GaN contact layers 157 and 257 and part ofthe p-type GaN/AlGaN superlattice cladding layers 156 and 256 areremoved by RIE method, and ridge stripe structures (a ridge section 158and the ridge section 258) are formed. On both sides of the ridgesections 158 and 258, a laminated insulating film 159 and the laminatedinsulating film 259 composed of SiO₂/Si is formed. The SiO₂ layer is thelower layer and the Si layer is the upper layer. The difference betweenthe effective refractive index of the ridge sections 158 and 258 and theeffective refractive index of the laminated insulating films 159 and 259is from 5×10⁻³ to 1×10⁻² both inclusive, and is specifically 7×10⁻³. Onthe p-type GaN contact layers 157 and 257 corresponding to the top faceof the ridge sections 158 and 258, the second electrodes (p-side ohmicelectrodes) 162 and 262 are formed. Meanwhile, on the rear face of then-type GaN substrates 121 and 221, a first electrode (n-side ohmicelectrode) 161 and the first electrode (n-side ohmic electrode) 261composed of Ti/Pt/Au are formed. Specifically, the laminated insulatingfilms 159 and 259 have SiO₂/Si laminated structure.

In the mode locking laser diode device 110 of the second example, thep-type AlGaN electron barrier layer 153, the p-type GaN/AlGaNsuperlattice cladding layer 156, and the p-type GaN contact layer 157that are Mg-doped compound semiconductor layers are arranged not tooverlap with each other as much as possible in the light densitydistribution generated from the third compound semiconductor layer 140and regions in the vicinity thereof. Theref, internal loss is inhibitedin a range in which internal quantum efficiency is not lowered.Therefore, threshold current I_(th) at which laser oscillation isstarted is decreased. Further, it was found that internal loss α_(i) islowered by increasing a value of distance d from the third compoundsemiconductor layers 140 to the p-type AlGaN electron barrier layer 153.It was also found that if the value d becomes a certain value or more,efficiency of hole injection into the well layer is lowered, and as aresult, electron-hole recombination ratio in the third compoundsemiconductor layer 140 is lowered, and internal quantum efficiencyη_(i) is decreased. Thus, the distance d from the third compoundsemiconductor layer 140 to the p-type AlGaN electron barrier layer 153was set to 0.10 μm, the height of the ridge section (ridge stripestructure) was set to 0.30 μm, the thickness of the second compoundsemiconductor layer 150 located between the second electrode 162 and thethird compound semiconductor layer 140 was set to 0.50 μm, and thethickness of a portion of the p-type GaN/AlGaN superlattice claddinglayer 156 located below the second electrode 162 was set to 0.40 μm.“The distance d between the electron barrier layer 153 and the thirdcompound semiconductor layers 140” means a distance between a portion ofthe electron barrier layer 153 facing the third compound semiconductorlayer 140 (interface) and a portion of the third compound semiconductorlayer 140 facing the electron barrier layer 153 (interface). Acomposition and a configuration of the semiconductor optical amplifier200 are similar to the foregoing composition and the foregoingconfiguration of the mode locking laser diode device 110.

In the second example, the second electrodes 162 and 262 are formed froma Pd single layer having a thickness of 0.1 μm. Further, in the secondexample, the width of an isolation trench 162C that separates the secondelectrode 162 composing the mode locking laser diode device 110 into afirst section 162A and a second section 162B is 1 μm or more and 50% orless the resonator length. Further, the length of a saturable absorptionregion 142 is shorter than the length of the light emitting region 141.Further, the length of the second electrode 162 (total length of thefirst section and the second section) is shorter than the length of thethird compound semiconductor layer 140. Specifically, resonator lengthZ″ was set to 0.60 mm, the length of the first section 162A of thesecond electrode 162 was set to 0.52 mm, the length of the secondsection 162B was set to 0.06 mm, and the width of the isolation trench162C (length in the resonator length direction) was set to 0.02 mm.

The thickness of the p-type GaN/AlGaN superlattice cladding layers 156and 256 having a superlattice structure in which a p-type GaN layer anda p-type AlGaN layer are alternately layered is 0.7 μm or less, andspecifically 0.4 μm. The thickness of the p-type GaN layer composing thesuperlattice structure is 2.5 nm. The thickness of the p-type AlGaNlayer composing the superlattice structure is 2.5 nm. The total numberof layers of the p-type GaN layer and the p-type AlGaN layer is 160.Further, the distance from the third compound semiconductor layers 140and 240 to the second electrodes 162 and 262 is 1 μm or less, andspecifically 0.5 μm. Further, the p-type AlGaN electron barrier layers153 and 253, the p-type GaN/AlGaN superlattice cladding layers 156 and256, and the p-type GaN contact layers 157 and 257 composing the secondcompound semiconductor layers 150 and 250 are doped with Mg at the levelof 1×10¹⁹ cm⁻³ or more (specifically at the level of 2×10¹⁹ cm⁻³).Further, the second compound semiconductor layers 150 and 250 areprovided with the non-doped compound semiconductor layer (the non-dopedGaInN light guide layers 151 and 251 and the non-doped AlGaN claddinglayer 152 and 252) and the p-type compound semiconductor layer from thethird compound semiconductor layer side. The distance d from the thirdcompound semiconductor layer 140 to the p-type compound semiconductorlayer (specifically, the p-type AlGaN electron barrier layers 153 and253) is 1.2×10⁻⁷ m or less, and specifically 100 nm.

Further, in the second example, a given value of voltage (voltage equalto or less than built-in voltage) is applied to the second section 262Bof the semiconductor optical amplifier 200 while laser light enters thesemiconductor optical amplifier 200 from the laser light source 100. Therelative position of the semiconductor optical amplifier 200 withrespect to laser light entering the semiconductor optical amplifier 200is adjusted so that the current flown in the semiconductor opticalamplifier 200 becomes the maximum.

Specifically, in the second example, when a current flown in the secondsection 262B of the semiconductor optical amplifier 200 in the casewhere a given value of voltage V₀ is applied to the semiconductoroptical amplifier 200 while laser light does not enter the semiconductoroptical amplifier 200 from the laser light source 100 is I₁, and acurrent flown in the second section 262B of the semiconductor opticalamplifier 200 in the case where a given value of voltage V₀ is appliedto the semiconductor optical amplifier 200 while laser light enters thesemiconductor optical amplifier 200 from the laser light source 100 isI₂, the relative position of the semiconductor optical amplifier 200with respect to laser light entering the semiconductor optical amplifier200 is adjusted so that value of ΔI=(I₂−I₁) becomes the maximum.

FIG. 13 schematically illustrates change of the current ΔI flown in thesemiconductor optical amplifier 200 in the case where a given value ofvoltage is applied to the semiconductor optical amplifier 200 whilelaser light enters the semiconductor optical amplifier 200 from thelaser light source 100 and the XYZ stage is moved in the X direction.Along with movement of the XYZ stage in the X direction, the current ΔIflown in the semiconductor optical amplifier 200 is flatly increaseduntil the ΔI exceeds the maximum value, and is flatly decreased. Changeof light output of laser light emitted from the semiconductor opticalamplifier 200 at this time shows the totally same behavior as that ofchange of current. Thus, light output of laser light emitted from thesemiconductor optical amplifier 200 is able to be the maximum byadjusting the relative position of the semiconductor optical amplifier200 with respect to laser light entering the semiconductor opticalamplifier 200 so that the current flown in the semiconductor opticalamplifier 200 becomes the maximum.

In the semiconductor optical amplifier 200 of the second example, in thecase where a given value of voltage V₀ is applied to the semiconductoroptical amplifier 200 while laser light enters the semiconductor opticalamplifier 200 from the laser light source 100 and the XYZ stage is movedin the X direction, as illustrated in FIG. 13, a voltage applied to(added to) the semiconductor optical amplifier 200 is increased. In thecase where the XYZ stage is moved, light output from the semiconductoroptical amplifier 200 is increased, and when such a phenomenon isgenerated, the number of carriers in the light amplification region(carrier injection region, gain region) 241 is decreased. Thus, acurrent flown in the semiconductor optical amplifier 200 is increased tocompensate such decrease of the number of carriers. Positioning methodof the semiconductor optical amplifier and the light output device inthe second example are based on the foregoing phenomenon. A positioningequipment (XYZ stage) 300 may be moved by an operator. Otherwise, thepositioning equipment (XYZ stage) 300 is able to be automatically movedby direction of a semiconductor optical amplifier control device 400based on voltage measurement result.

In the second example, a current applied to the semiconductor opticalamplifier 200 is measured to adjust the relative position of thesemiconductor optical amplifier 200 with respect to laser light enteringthe semiconductor optical amplifier 200. Thus, measurement for positionadjustment is able to be performed without depending an externalmonitoring equipment. Thus, the relative position of the semiconductoroptical amplifier 200 with respect to laser light entering thesemiconductor optical amplifier 200 is able to be adjusted accurately.

Further, by monitoring the currents I₁ and I₂ flown in the secondsection 262B of the semiconductor optical amplifier 200, operation stateof the semiconductor optical amplifier 200 and the mode locking laserdiode device 110 is able to be monitored.

In the mode locking laser diode device 110 of the second example, thethird compound semiconductor layer 140 includes the saturable absorptionregion 142. Further, the second electrode 162 is separated into thefirst section 162A for obtaining forward bias state by flowing a currentto the first electrode 161 through the light emitting region 141, andthe second section 162B for applying an electric field to the saturableabsorption region 142 by the isolation trench 162C. Forward bias stateis obtained by flowing a current to the first electrode 161 from thefirst section 162A of the second electrode 162 through the lightemitting region 141, and an electric field is added to the saturableabsorption region 142 by applying a voltage between the first electrode161 and the second section 162B of the second electrode 162. Inaddition, in the mode locking laser diode device of the second example,light pulse is generated in the light emitting region 141 by flowing acurrent from the second electrode 162 to the first electrode 161 throughthe laminated structure.

Specifically, in the mode locking laser diode device 110 of the secondexample, as described above, the second electrode 162 is separated intothe first section 162A for obtaining forward bias state by flowing adirect current (forward bias current I_(gain)) to the first electrode161 through the light emitting region (gain region) 141, and the secondsection 162B for applying an electric field to the saturable absorptionregion 142 (the second section 162B for adding reverse bias voltageV_(sa) to the saturable absorption region 142) by the isolation trench162C. The electric resistance value (also referred to as “separatingresistance value”) between the first section 162A and the second section162B of the second electrode 162 is 1×10 times or more the electricresistance value between the second electrode 162 and the firstelectrode 161, is specifically 1.5×10³ times the electric resistancevalue between the second electrode 162 and the first electrode 161.Further, the electric resistance value (separating resistance value)between the first section 162A and the second section 162B of the secondsection 162 is 1×10²Ω or more, and is specifically 1.5×10⁴Ω.

Further, in the mode locking laser diode device 110 of the secondexample, the second electrode 162 having a separating resistance valueof 1×10²Ω or more should be formed on the second compound semiconductorlayer 150. In the case of the GaN laser diode device, mobility in thecompound semiconductor having p-type conductivity type is smalldifferently in the existing GaAs laser diode device. Thus, it ispossible that the electric resistance value between the first section162A and the second section 162B of the second electrode 162 becomes 10times or more the electric resistance value between the second electrode162 and the first electrode 161, or the electric resistance valuebetween the first section 162A and the second section 162B of the secondsection 162 becomes 1×10²Ω or more without setting high resistance ofthe second compound semiconductor layer 150 having p-type conductivitytype by ion injection or the like but separating the second electrode162 formed thereon by the isolation trench 162C.

A description will be given of a method of manufacturing the modelocking laser diode device of the second example with reference to FIGS.29A, 29B, 30A, 30B, and 31. FIGS. 29A, 29B, 30A, and 30B are schematicpartial cross sectional views where the substrate and the like are cutin XY plane. FIG. 31 is a schematic partial end view where the substrateand the like are cut in YZ plane.

Requested characteristics of the second electrode 162 are as follows:

(1) a function as an etching mask in etching the second compoundsemiconductor layer 150 is included;

(2) the second electrode 162 is able to be wet-etched withoutdeteriorating optical and electric characteristics of the secondcompound semiconductor layer 150;

(3) contact specific resistance value of 10⁻²Ω·cm² or less is shown inthe case where the second electrode 162 is formed on the second compoundsemiconductor layer 150;

(4) in the case of a laminated structure, a material composing the lowermetal layer has large work function, shows low contact specificresistance value to the second compound semiconductor layer 150, and isable to be wet-etched; and

(5) in the case of a laminated structure, a material composing the uppermetal layer has resistance to etching in forming the ridge stripestructure (for example, Cl₂ gas used in RIE method), and is able to bewet-etched.

Step-200

First, a laminated structure in which the first compound semiconductorlayer 130 that has first conductivity type (n-type conductivity type)and is composed of GaN compound semiconductor, the third compoundsemiconductor layer (active layer) 140 including the light emittingregion (gain region) 141 composed of GaN compound semiconductor and thesaturable absorption region 142, and the second compound semiconductorlayer 150 that has second conductivity type (p-type conductivity type)different from the first conductivity type and is composed of GaNcompound semiconductor are sequentially layered is formed on asubstrate, specifically on (0001) plane of the n-type GaN substrate 121based on known MOCVD method (refer to FIG. 29A).

Step-210

After that, the strip-shaped second electrode 162 is formed on thesecond compound semiconductor layer 150. Specifically, after a Pd layer163 is formed over the entire face of the second compound semiconductorlayer 150 based on vacuum evaporation method (refer to FIG. 29B), astrip-shaped etching-use resist layer is formed on the Pd layer 163based on photolithography technique. After the Pd layer 163 not coveredwith the etching-use resist layer is removed by using aqua regia, theetching-use resist layer is removed. Thereby, the structure illustratedin FIG. 30A is able to be obtained. It is possible that the strip-shapedsecond electrode 162 is formed on the second compound semiconductorlayer 150 based on liftoff method.

Step-220

Next, at least part of the second compound semiconductor layer 150 isetched (in the second example, part of the second compound semiconductorlayer 150 is etched) with the use of the second electrode 162 as anetching-use mask to form the ridge stripe structure. Specifically, partof the second compound semiconductor layer 150 is etched with the use ofthe second electrode 162 as an etching-use mask based on RIE methodusing Cl₂ gas. Thereby, the structure illustrated in FIG. 30B is able tobe obtained. As described above, the ridge stripe structure is formed byself alignment method by using the second electrode 162 patterned in theshape of a strip as an etching-use mask. Thus, misalignment is notgenerated between the second electrode 162 and the ridge stripestructure.

Step-230

After that, a resist layer 164 for forming the isolation trench in thesecond electrode 162 is formed (refer to FIG. 31). Referential number165 represents an aperture provided in the resist layer 164 for formingthe isolation trench. Next, the isolation trench 162C is formed in thesecond electrode 162 by wet etching method with the use of the resistlayer 164 as a wet etching-use mask, and thereby the second electrode162 is separated into the first section 162A and the second section 162Bby the isolation trench 162C. Specifically, aqua regia is used as anetching liquid, and the entire body is dipped into the aqua regia forabout 10 seconds, and thereby the isolation trench 162C is formed in thesecond electrode 162. After that, the resist layer 164 is removed.Accordingly, the structure illustrated in FIG. 10 is able to beobtained. As described above, differently from dry etching method, byadopting wet etching method, optical characteristics and electriccharacteristics of the second compound semiconductor layer 150 are notdeteriorated. Thus, light emitting characteristics of the mode lockinglaser diode device are not deteriorated. If dry etching method isadopted, there is a possibility that internal loss α_(i) of the secondcompound semiconductor layer 150 is increased, the threshold voltage isincreased, and light output is lowered. In this case, when an etchingrate of the second electrode 162 is ER₀, and an etching rate of thelaminated body is ER₁, the following formula is established:ER₀/ER_(1≈)1×10²As described above, since the high etching selection ratio existsbetween the second electrode 162 and the second compound semiconductorlayer 150, the second electrode 162 is able to be surely etched withoutetching the laminated structure (or even if the laminated structure isetched, the etching amount is slight).

Step-240

After that, the n-side electrode 161 is formed, the substrate iscleaved, and further packaging is made. Accordingly, the mode lockinglaser diode device 110 is able to be fabricated.

In general, resistance R (Ω) of a semiconductor layer is expressed asfollows by using specific resistance value ρ (Ω·m) of a materialcomposing a semiconductor layer, length of the semiconductor layer X₀(m), cross section area S of the semiconductor layer (m²), carrierdensity n (cm⁻³), electric charge amount e (C), and mobility μ (m²/Vsec).R=(ρ·X₀)/S=X₀/(n·e·μ·S)

Since mobility of the p-type GaN semiconductor is two-digit or moresmaller than that of the p-type GaAs semiconductor, the electricresistance value gets high easily. Thus, it is found that the electricresistance value of the laser diode device having a ridge stripestructure with small cross section area being 1.5 μm wide and 0.35 μmhigh becomes a large value based on the foregoing formula.

FIG. 27 illustrates a result obtained by measuring an electricresistance value between the first section 162A and the second section162B of the second electrode 162 of the fabricated mode locking laserdiode device 110 of the second example by four terminal method. When thewidth of the isolation trench 162C was 20 μm, the electric resistancevalue between the first section 162A and the second section 162B of thesecond electrode 162 was 15 kΩ.

In the fabricated mode locking laser diode device 110 of the secondexample, forward bias state was obtained by flowing a direct currentfrom the first section 162A of the second electrode 162 to the firstelectrode 161 through the light emitting region 141, and electric fieldwas applied to the saturable absorption region 142 by applying reversebias voltage V_(sa) between the first electrode 161 and the secondsection 162B of the second electrode 162, and thereby mode locking drivewas performed.

Further, the electric resistance value between the first section 162Aand the second section 162B of the second electrode 162 is ten times ormore the electric resistance value between the second electrode 162 andthe first electrode 161, or 1×10²Ω or more. Thus, flow of leakagecurrent from the first section 162A of the second electrode 162 to thesecond section 162B of the second electrode 162 is able to be inhibitedsecurely. In the result, the light emitting region 141 is able to be inforward bias state, the saturable absorption region 142 is securely ableto be in reverse bias state, and mode locking operation is able to besecurely performed.

Further, the semiconductor optical amplifier 200 is able to bemanufactured by the same manufacturing method as that of the modelocking laser diode device 110, except that the structure of the secondelectrode is different. Thus, detailed description thereof will beomitted.

To promote better understanding of the mode locking laser diode deviceof the second example, a mode locking laser diode device of a secondreferential example was fabricated. In the mode locking laser diodedevice of the second referential example, the structure of the thirdcompound semiconductor layer 140 in the layer structure illustrated inTable 2 was as illustrated in the following Table 3.

TABLE 3 Second referential Second example example Well layer 8 nm 10.5nm Barrier layer 12 nm    14 nm Impurity doping concentration Non-dopedNon-doped of well layer Impurity doping concentration Si: 2 × 10¹⁸ cm⁻³Non-doped of barrier layer

In the second example, the thickness of the well layer is 8 nm, thebarrier layer is doped with Si at a concentration of Si: 2×10¹⁸ cm⁻³,and QCSE effect in the third compound semiconductor layer is modified.Meanwhile, in the second referential example, the thickness of the welllayer is 10.5 nm, and the barrier layer is not doped with impurity.

A light condensing external resonator was formed from the mode lockinglaser diode devices of the second example and the second referentialexample, and mode locking driving was performed (refer to FIG. 19A). Inthe light condensing external resonator illustrated in FIG. 19A, theexternal resonator is configured of the end face of the mode lockinglaser diode device in which a high reflective coating layer (HR) isformed on the saturable absorption region side and the external mirror13, and light pulse is extracted from the external mirror 13. A lowreflective coating layer (AR) is formed on the end face (light outputend face) of the mode locking laser diode device on the light emittingregion (gain region) side. As the optical filter 12, a bandpass filteris mainly used, which is inserted for controlling laser oscillationwavelength. Repetition frequency f of light pulse train is determined bythe external resonator length Z′ as expressed by the following formula,where c represents light velocity and n represents reflective index ofwaveguide.f=c/(2n·Z′)

Mode locking is determined by a direct current applied to the lightemitting region 141 and the reverse bias voltage V_(sa) applied to thesaturable absorption region 142. FIGS. 25A and 25B illustrate reversebias voltage dependence measurement results of relation between aninjection current and light output (L-I characteristics) of the secondexample and the second referential example. In FIGS. 25A and 25B,measurement results affixed with referential symbol “A” are results inthe case of the reverse bias voltage V_(sa)=0 volt, measurement resultsaffixed with referential symbol “B” are results in the case of thereverse bias voltage V_(sa)=−3 volt, measurement results affixed withreferential symbol “C” are results in the case of the reverse biasvoltage V_(sa)=−6 volt, and measurement results affixed with referentialsymbol “D” are results in the case of the reverse bias voltage V_(sa)=−9volt. In FIG. 25A, the measurement result in the case of the reversebias voltage V_(sa)=0 volt almost overlaps the measurement result in thecase of the reverse bias voltage V_(sa)=−3 volt.

Based on comparison between FIGS. 25A and 25B, it is found that in thesecond referential example, as the reverse bias voltage V_(sa) isincreased, the threshold current I_(th) at which laser oscillation isstarted is gradually increased, and change is shown at lower reversebias voltage V_(sa) compared to in the second example. It indicates thatin the third compound semiconductor layer 140 of the second example,effect of saturable absorption is electrically controlled more by thereverse bias voltage V_(sa).

FIGS. 26A and 26B illustrate results obtained by measuring light pulsegenerated in the second example and the second referential example by astreak camera. In FIG. 26B obtained in the second referential example,subpulse component is generated before and after main pulse. Meanwhile,in FIG. 26A obtained in the second example, subpulse component isinhibited from being generated. The results may be all caused byincreased effect of saturable absorption since QCSE effect is moderatedby the structure of the third compound semiconductor layer 140.

Drive conditions and the like of the mode locking laser diode device ofthe second example illustrated in FIG. 19A are exemplified in thefollowing Table 4. I_(th) represents a threshold current.

TABLE 4 Mode locking drive conditions: 0 < 1_(gain)/I_(th) ≦ 5 −20 ≦V_(sa) (volt) ≦ 0 High reflective coating layer (HR): 85 ≦ reflectanceR_(HR) (%) < 100 Low reflective coating layer (AR): 0 < reflectanceR_(AR) (%) ≦ 0.5 Optical filter: 85 ≦ transmittance T_(BPF) (%) < 100 0< half bandwidth τ_(BPF) (nm) ≦ 2.0 400 < peak wavelength λ_(bpf) (nm) <450 External mirror: 0 < reflectance Roc (%) < 100 External resonatorlength Z′ 0 < Z′ (mm) < 1500

More specifically, in the second example, the following conditions wereadopted as an example:

-   I_(gain): 120 mA-   I_(th): 45 mA-   Reverse bias voltage V_(sa): −11 (volt)-   Reflectance R_(HR): 95%-   Reflectance R_(AR): 0.3%-   Transmittance T_(BPF): 90%-   Half bandwidth τ_(BPF): 1 nm-   Peak wavelength λ_(BPF): 410 nm-   Reflectance R_(OC): 20%-   External resonator length Z′: 150 mm

Meanwhile, in the second referential example, the same conditions asthose of the second example were adopted except for the followingconditions:

-   I_(gain): 95 mA-   I_(th): 50 mA-   Reverse bias voltage V_(sa): −12.5 (volt)-   Reflectance R_(OC): 50%

As illustrated in the conceptual view of FIG. 14A, part of light outputof laser light outputted from the semiconductor optical amplifier 200 isextracted by using a beam splitter 32, and extracted light enters aphotodiode 34 through a lens 33. Thereby, the light output of laserlight outputted from the semiconductor optical amplifier 200 may bemeasured. In the case where light output is changed from a desiredvalue, alignment method of the semiconductor optical amplifier of thesecond example is executed again. That is, a given value of voltage V₀is applied to the semiconductor optical amplifier 200 while laser lightenters the semiconductor optical amplifier 200 from the laser lightsource 100, and thereby the relative position of the semiconductoroptical amplifier 200 with respect to laser light entering thesemiconductor optical amplifier 200 is adjusted again so that a currentflown in the semiconductor optical amplifier 200 becomes the maximum. Inthe case where result of readjustment of the relative position of thesemiconductor optical amplifier 200 with respect to laser light enteringthe semiconductor optical amplifier 200 is the same as the relativeposition of the semiconductor optical amplifier with respect to laserlight entering the semiconductor optical amplifier 200 beforereadjustment, light path through which the laser light outputted fromthe semiconductor optical amplifier 200 passes is adjusted. Suchadjustment may be performed by, for example, laying a reflective mirror31 on an XYZ stage 35. The XYZ stage 35 may be moved by an operator.Otherwise, the XYZ stage 35 is able to be automatically moved bydirection of the semiconductor optical amplifier control device 400based on the voltage and measurement result of the photodiode 34. InFIG. 14A, elements of the light output device located in the upstream ofthe semiconductor optical amplifier 200 are the same as the elements ofthe light output device of the second example, and thus the elements ofthe light output device located in the upstream of the semiconductoroptical amplifier 200 are not illustrated in the figure. By adoptingsuch a method, in the case where change occurs in the light outputmonitor, it is possible to easily determine whether or not such changeis caused by relative position change of the semiconductor opticalamplifier 200 with respect to laser light entering the semiconductoroptical amplifier 200 (that is, change of efficiency of coupling of theentrance laser light and the light guide of the semiconductor opticalamplifier).

Third Example

The third example relates to the semiconductor optical amplifiersaccording to the second embodiment and the third embodiment of theinvention. FIGS. 15A and 16 illustrate a schematic perspective view ofthe semiconductor optical amplifier and a schematic plan view of a ridgestripe structure according to the second embodiment of the invention ofthe third example. The width of the second electrode 262 is narrowerthan the width of the ridge stripe structure. In this case, (width ofthe second electrode)/(width of the ridge stripe structure) satisfies avalue from 0.2 to 0.9 both inclusive. Further, FIGS. 17A and 18illustrate a schematic perspective view of the semiconductor opticalamplifier and a schematic plan view of a ridge stripe structureaccording to the third embodiment of the invention of the third example.Where the maximum width of the ridge stripe structure is W_(max),W_(max)/W_(out) is satisfied, and 0.2≦W_(out)/W_(max)≦0.9 is satisfied.In FIG. 18, though the second electrode 262 is not illustrated, thesecond electrode 262 is formed from the p-type GaN contact layercorresponding to the top face of the ridge section to part of the topface of the p-type AlGaN cladding layer as in the first example.

A composition and a structure of the semiconductor optical amplifier ofthe third example are the same as the composition and the structure ofthe semiconductor optical amplifier described in the first exampleexcept for the foregoing points or except that the carrier non-injectionregion is not provided, and thus detailed description thereof will beomitted.

As illustrated in FIG. 6B, in the case where the width of the near-fieldimage is narrower than W_(out), there is a possibility that light fieldbecomes unstable depending on drive conditions and light outputconditions such as the light density, the carrier diffusion length, anddevice temperature. Thus, in the third example, by adopting theforegoing composition and the foregoing structure, mode instability ismodified.

Fourth Example

The fourth example is a modification of the third example. FIG. 15Billustrates a schematic perspective view of a modified example of thesemiconductor optical amplifier illustrated in FIGS. 15A and 16, andFIG. 17B illustrates a schematic perspective view of a modified exampleof the semiconductor optical amplifier illustrated in FIGS. 17A and 18.As illustrated in FIG. 15B and FIG. 17B, in the fourth example,differently from the third example, a carrier non-injection region isprovided in the internal region of the laminated structure from thelight output end face along the axis line of the semiconductor opticalamplifier. A composition and a structure of the semiconductor opticalamplifier of the fourth example are the same as the composition and thestructure of the semiconductor optical amplifier described in the thirdexample except for the foregoing points, and thus detailed descriptionthereof will be omitted. In the forth example, the second electrode maybe separated into the first section and the second section by theisolation trench as in the second example.

Fifth Example

The fifth example is a modification of the mode locking laser diodedevice in the second example. FIGS. 19B, FIG. 20A, and FIG. 20Billustrate an example in which an external resonator is structured bythe mode locking laser diode device of the fifth example.

In the collimation type external resonator illustrated in FIG. 19B, theexternal resonator is formed from the end face of the mode locking laserdiode device in which a high reflective coating layer (HR) is formed onthe saturable absorption region side and the external mirror, and lightpulse is extracted from the external mirror. A low reflective coatinglayer (AR) is formed on the end face (light output end face) of the modelocking laser diode device on the light emitting region (gain region)side. The drive conditions and the like of the mode locking laser diodedevice of the fifth example illustrated in FIG. 19B are similar to thoseof the foregoing Table 4.

Meanwhile, in the external resonator illustrated in FIGS. 20A and 20B,the external resonator is formed from the end face of the mode lockinglaser diode device in which a reflective coating layer (R) is formed onthe saturable absorption region side (light output end face) and theexternal mirror, and light pulse is extracted from the saturableabsorption region 142. A low reflective coating layer (AR) is formed onthe end face of the mode locking laser diode device on the lightemitting region (gain region) side. The example illustrated in FIG. 20Ais light condensing type, and the example illustrated in FIG. 20B iscollimation type. The drive conditions and the like of the mode lockinglaser diode device of the fifth example illustrated in FIGS. 20A and 20Bare similar to those of the foregoing Table 4. However, the reflectivecoating layer (R) may be as illustrated in the following Table 5.

TABLE 5 Reflective coating layer (R) 0 < reflectance R_(R) (%) < 100

Specifically, reflectance R_(R) was set to 20%. A composition and astructure of the mode locking laser diode device in the fifth exampleare the same as the composition and the structure of the mode lockinglaser diode device described in the second example, and thus detaileddescription thereof will be omitted.

Sixth Example

The sixth example is also a modification of the mode locking laser diodedevice of the second example. In the sixth example, as illustrated inFIG. 20C, the mode locking laser diode device is monolithic type. Thedrive conditions and the like of the mode locking laser diode device ofthe sixth example are similar to those of the foregoing Table 4. Othercomposition and other structure of the mode locking laser diode deviceof the sixth example are similar to the composition and the structure ofthe mode locking laser diode device described in the second example, andthus detailed description thereof will be omitted.

Seventh Example

The seventh example is also a modification of the mode locking laserdiode device in the second example. The mode locking laser diode deviceof the seventh example is a laser diode device having a ridge stripetype separate confinement heterostructure with oblique light guide. FIG.21 illustrates a schematic view viewed from above of a ridge section158A in the mode locking laser diode device of the seventh example. Themode locking laser diode device of the seventh example has a structurein which two straight line-like ridge sections. A value of angle θ′ ofintersection of the two ridge sections desirably satisfies, for example,0<θ′≦10 (deg), and preferably satisfies 0<θ′≦6 (deg). By adopting theoblique ridge stripe type, reflectance of the end face provided with lowreflective coating is able to be closer to 0% as the ideal value. In theresult, generation of light pulse that would revolve in the laser diodedevice is able to be prevented, and generation of sub-light pulseassociated with main light pulse is able to be inhibited. The obliqueridge stripe type mode locking laser diode device of the seventh exampleis applicable to the second example, the fifth example, and the sixthexample. Other composition and other structure of the mode locking laserdiode device in the seventh example are similar to the composition andthe structure of the mode locking laser diode device described in thesecond example, and thus detailed description thereof will be omitted.

Eighth Example

The eighth example is also a modification of the mode locking laserdiode device in the second example. In the eighth example, a current isflown from the second electrode 162 to the first electrode 161 throughthe light emitting region 141, and an external electric signal (RMSjitter Δ_(signal)) is superimposed on the first electrode 161 from thesecond electrode 162 through the light emitting region 141. FIG. 22Aschematically illustrates a system of performing mode locking drive byusing the mode locking laser diode device of the eighth example. Theexternal electric signal is sent from a known external electric signalgenerator to the second electrode 162. Thereby, light pulse is able tobe sync with the external electric signal. That is, RMS timing jitterΔt_(MILD) is able to be kept down as the following formula:Δ_(signal)≦Δt_(MILD).

The drive conditions and the like of the mode locking laser diode deviceof the eighth example illustrated in FIG. 22A are similar to those ofthe foregoing Table 4. Voltage maximum value V_(p-p) (unit: volt) of theexternal electric signal desirably satisfies 0<V_(p-p)≦10, andpreferably satisfies 0<V_(p-p)≦3. Further, frequency f_(signal) of theexternal electric signal and repetition frequency f_(MILD) of a lightpulse train desirably satisfy 0.99≦f_(signal)/f_(MILD)≦1.01.

More specifically, in the eighth example, the following conditions wereadopted as an example:

-   I_(gain): 120 mA-   I_(th): 45 mA-   Reverse bias voltage V_(sa): −11 (volt)-   Reflectance R_(HR): 95%-   Reflectance R_(AR): 0.3%-   Transmittance T_(BPF): 90%-   Half bandwidth τ_(BPF): 1 nm-   Peak wavelength λ_(BPF): 410 nm-   Reflectance R_(OC): 20%-   External resonator length Z′: 150 mm-   V_(p-p): 2.8 volt-   f_(signal): 1 GHz-   f_(MILD): 1 GHz-   Δ_(signal): 1 picosecond-   Δt_(MILD): 1.5 picosecond

Meanwhile, in the eighth referential example, a current was flown fromthe second electrode 162 to the first electrode 161 through the lightemitting region 141 without superimposing an external electric signal onthe first electrode 161 from the second electrode 162 through the lightemitting region 141. RF spectrum was measured. FIGS. 28A and 28Billustrate measurement results in the eighth example and the eighthreferential example. In the eighth referential example, the sameconditions as those of the eighth example were adopted except for thefollowing conditions:

Reflectance R_(OC): 50%

FIGS. 28A and 28B show that in the eighth example, the area of bottomcomponent of RF spectrum is decreased more than in the eighthreferential example. Such a fact shows that the eighth example is adrive method in which the phase noise and the timing jitter are smallercompared to those of the eighth referential example.

Other composition and other structure of the mode locking laser diodedevice in the eighth example are similar to the composition and thestructure of the mode locking laser diode device described in the secondexample, the fifth example, the sixth example, and the seventh example,and thus detailed description thereof will be omitted.

Ninth Example

The ninth example is also a modification of the mode locking laser diodedevice in the second example. In the ninth example, an optical signalenters from one end face of the laminated structure. FIG. 22Bschematically illustrates a system of performing mode locking drive byusing the mode locking laser diode device of the ninth example. Theoptical signal (RMS jitter: Δ_(opto)) is outputted from an opticalsignal generator composed of the laser diode device, and enters one endface of the laminated structure through a lens, an external mirror, anoptical filter, and a lens. Thereby, light pulse is able to be sync withthe optical signal. That is, the RMS timing jitter Δt_(MILD) is able tobe kept down as the following formula. Δ_(opto)≦Δt_(MILD).

Other composition and other structure of the mode locking laser diodedevice in the ninth example are similar to the composition and thestructure of the mode locking laser diode device described in the secondexample, the fifth example, the sixth example, and the seventh example,and thus detailed description thereof will be omitted.

Descriptions have been hereinbefore given of the invention withreference to the preferred embodiments. However, the invention is notlimited to the foregoing embodiments. The compositions and thestructures of the semiconductor optical amplifier, the light outputdevice, the laser light source, and the laser diode device described inthe embodiments are just exemplified, and modifications may be made asappropriate. Further, in the embodiments, though various values havebeen shown, such various values are just exemplified as well, and thusit is needless to say that, for example, if specifications of thesemiconductor optical amplifier, the light output device, and the laserdiode device to be used are changed, values are also changed. Forexample, the second electrode 162 may have a laminated structureincluding a lower metal layer composed of palladium (Pd) having athickness of 20 nm and an upper metal layer composed of nickel (Ni)having a thickness of 200 nm. In performing wet etching with the use ofaqua regia, the etching rate of nickel is about 1.25 times the etchingrate of palladium.

In the embodiments, the semiconductor optical amplifier is composed of atransmissive semiconductor optical amplifier. However, the semiconductoroptical amplifier is not limited thereto. As illustrated in a conceptualview of FIG. 14B, the semiconductor optical amplifier may be composed ofa monolithic semiconductor optical amplifier. The monolithicsemiconductor optical amplifier is an integrated body composed of alaser diode device and a semiconductor optical amplifier.

In the embodiments, the mode locking laser diode device 110 is providedon the {0001} plane, which is the C plane as the polarity plane of then-type GaN substrate 121. Alternately, the mode locking laser diodedevice 110 may be provided on A plane as {11-20} plane, M plane as{1-100} plane, non-polarity plane such as {1-102} plane, {11-2n} planeincluding {11-24} plane and {11-22} plane, or a semi-polarity plane suchas {10-11} plane and {10-12} plane. Even if piezoelectric polarizationor intrinsic polarization is thereby generated in the third compoundsemiconductor layer of the mode locking laser diode device 110,piezoelectric polarization is not generated in the thickness directionof the third compound semiconductor layer and piezoelectric polarizationis generated in the direction approximately perpendicular to thethickness direction of the third compound semiconductor layer. Thus,adverse effect resulting from piezoelectric polarization and intrinsicpolarization is able to be excluded. {11-2n} plane means a non-polarityplane making 40 deg approximately with respect to the C plane. In thecase where the mode locking laser diode device 110 is provided on anon-polarity plane or on a semi-polarity plane, limitation of thethickness of the well layer (from 1 nm to 10 nm both inclusive) andlimitation of the impurity doping concentration of the barrier layer(from 2×10¹⁸ cm⁻³ to 1×10²⁰ cm⁻³ both inclusive) are able to beeliminated.

The number of the light emitting regions 141 and the saturableabsorption regions 142 is not limited to 1. FIG. 23 illustrates aschematic end view of a mode locking laser diode device in which onefirst section 162A of the second electrode and two second sections162B₁, and 162B₂ of the second electrode are provided. In the modelocking laser diode device, one end of the first section 162A is opposedto one second section 162B₁ with one isolation trench 162 C₁ in between,and the other end of the first section 162A is opposed to the othersecond section 162B₂ with the other isolation trench 162C₂ in between.Further, one light emitting region 141 is sandwiched between saturableabsorption regions 142 ₁, and 142 ₂. Further, FIG. 24 illustrates aschematic end view of a mode locking laser diode device in which twofirst sections 162A₁, and 162A₂ of the second electrode and one secondsection 162B of the second electrode are provided. In the mode lockinglaser diode device, an end section of the second section 162B is opposedto one first section 162A₁ with one isolation trench 162 C₁ in between,and the other end of the second section 162B is opposed to the otherfirst section 162A₂ with the other isolation trench 162 C₂ in between.Further, one saturable absorption region 142 is sandwiched between twolight emitting regions 141 ₁, and 141 ₂.

Further, as a modification of the second embodiment, it is possible thata given value of current is applied to the semiconductor opticalamplifier while laser light enters the semiconductor optical amplifierfrom the laser light source, and thereby the relative position of thesemiconductor optical amplifier with respect to laser light entering thesemiconductor optical amplifier is adjusted so that voltage applied to(added to) the semiconductor optical amplifier becomes the maximum. Inthis case, in the case where light output of laser light outputted fromthe semiconductor optical amplifier is measured and the light output ischanged from a desired value, it is possible that a given value ofcurrent is applied to the semiconductor optical amplifier while laserlight enters the semiconductor optical amplifier from the laser lightsource, and thereby the relative position of the semiconductor opticalamplifier with respect to laser light entering the semiconductor opticalamplifier is adjusted again so that voltage applied to (added to) thesemiconductor optical amplifier becomes the maximum. Further, in thecase where result of readjustment of the relative position of thesemiconductor optical amplifier with respect to laser light entering thesemiconductor optical amplifier is the same as the relative position ofthe semiconductor optical amplifier with respect to laser light enteringthe semiconductor optical amplifier before readjustment, light paththrough which the laser light outputted from the semiconductor opticalamplifier passes is able to be adjusted. Specifically, where a voltageapplied to (added to) the semiconductor optical amplifier in the casewhere a given value of current I₀ is flown to the semiconductor opticalamplifier while laser light does not enter the semiconductor opticalamplifier from the laser light source is V₁, and a voltage applied to(added to) the semiconductor optical amplifier in the case where a givenvalue of current I₀ is flown to the semiconductor optical amplifierwhile laser light enters the semiconductor optical amplifier from thelaser light source is V₂, the relative position of the semiconductoroptical amplifier with respect to laser light entering the semiconductoroptical amplifier may be adjusted so that value of ΔV=(V₂−V₁) becomesthe maximum. As a given value of current, 0 milliampere<ΔI≦20milliampere is able to be exemplified.

Further, as a modification of the second embodiment, it is possible thata given value of voltage is applied to the semiconductor opticalamplifier while laser light enters the semiconductor optical amplifierfrom the laser light source, and thereby the relative position of thesemiconductor optical amplifier with respect to laser light entering thesemiconductor optical amplifier is adjusted so that a current flown tothe semiconductor optical amplifier becomes the maximum. In this case,when light output of laser light outputted from the semiconductoroptical amplifier is measured and the light output is changed from adesired value, it is possible that a given value of voltage is appliedto the semiconductor optical amplifier while laser light enters thesemiconductor optical amplifier from the laser light source, and therebythe relative position of the semiconductor optical amplifier withrespect to laser light entering the semiconductor optical amplifier isadjusted again so that current flown in the semiconductor opticalamplifier becomes the maximum. Further, in the case where result ofreadjustment of the relative position of the semiconductor opticalamplifier with respect to laser light entering the semiconductor opticalamplifier is the same as the relative position of the semiconductoroptical amplifier with respect to laser light entering the semiconductoroptical amplifier before readjustment, light path through which thelaser light outputted from the semiconductor optical amplifier passes isable to be adjusted. Specifically, when a current flown in thesemiconductor optical amplifier in the case where a given value ofvoltage V₀ is applied to the semiconductor optical amplifier while laserlight does not enter the semiconductor optical amplifier from the laserlight source is I₁, and a current flown in the semiconductor opticalamplifier in the case where a given value of voltage V₀ is applied tothe semiconductor optical amplifier while laser light enters thesemiconductor optical amplifier from the laser light source is I₂, therelative position of the semiconductor optical amplifier with respect tolaser light entering the semiconductor optical amplifier may be adjustedso that value of ΔI=(I₂−I₁) becomes the maximum. As a given value ofvoltage, 0 volt≦ΔV≦5 volt is able to be exemplified.

The present application contains subject matter related to thatdisclosed in Japanese Priority Patent Application JP 2010-149345 filedin the Japanese Patent Office on Jun. 30, 2010, the entire contents ofwhich is hereby incorporated by reference.

It should be understood by those skilled in the art that variousmodifications, combinations, sub combinations and alternations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

What is claimed is:
 1. A semiconductor optical amplifier comprising: alaminated structure including (i) a first compound semiconductor layerthat has a first conductivity type and is composed of GaN compoundsemiconductor, (ii) a second compound semiconductor layer that has asecond conductivity type different from the first conductivity type andis composed of GaN compound semiconductor, and (iii) a third compoundsemiconductor layer that has a light amplification region composed ofGaN compound semiconductor, the first compound semiconductor layer, thethird compound semiconductor layer, and the second compoundsemiconductor layer being sequentially layered; a second electrode onthe second compound semiconductor layer; and a first electrodeelectrically connected to the first compound semiconductor layer,wherein, the laminated structure has a ridge stripe structure, when awidth of the ridge stripe structure in a light output end face isW_(out), and a width of the ridge stripe structure in a light incidentend face is W_(in), W_(out)>W_(in) is satisfied, and a carriernon-injection region is provided in the laminated structure, the carriernon-injection region extending from the light output end face of theridge stripe structure and towards the light incident end face of theridge stripe structure along an axis line of the semiconductor opticalamplifier, the first, second, and third compound semiconductor layersbeing present in the carrier non-injection region, and the secondelectrode not being present in the carrier non-injection region.
 2. Thesemiconductor optical amplifier according to claim 1, wherein W_(out) isgreater or equal to 5 μm.
 3. The semiconductor optical amplifieraccording to claim 1, wherein W_(in) is from 1.4 μm to 2.0 μm, bothinclusive.
 4. The semiconductor optical amplifier according to claim 1,wherein the second electrode is composed of a first section and a secondsection separated by an isolation trench, and the second section of thesecond electrode is provided in the carrier non-injection region.
 5. Thesemiconductor optical amplifier according to claim 4, wherein a voltageequal to or less than a built-in voltage is applied to the secondsection of the second electrode.
 6. The semiconductor optical amplifieraccording to claim 1, wherein the axis line of the semiconductor opticalamplifier intersects with an axis line of the ridge stripe structure ata given angle that ranges from 0.1 to 10 degrees.
 7. The semiconductoroptical amplifier according to claim 1, wherein a value of (width of theridge stripe structure in the light output end face)/(width of laserlight outputted from the semiconductor optical amplifier) is from 1.1 to10, both inclusive.
 8. A light output device comprising: a laser lightsource; and a semiconductor optical amplifier according claim 1 thatoptically amplifies laser light from the laser light source and outputsamplified light.
 9. The light output device according to claim 8,further comprising a mirror and a light isolator positioned such thatlaser light output from the laser light source passes through the lightisolator and then is reflected by the mirror.
 10. The light outputdevice according to claim 9, further comprising a half-wave plate and alens positioned such that laser light reflected by the mirror passesthrough the half-wave plate and lens and then enters the semiconductoroptical amplifier.
 11. The light output device according to claim 10,wherein the light isolator is positioned to prevent light returned fromthe semiconductor optical amplifier from returning to the laser lightsource.
 12. The light output device according to claim 11, furthercomprising an output lens, wherein the laser light is opticallyamplified in the semiconductor optical amplifier, and is output outsidethe system through the output lens.
 13. The light output deviceaccording to claim 8, wherein the laser light source is a mode lockinglaser diode device, and pulse laser light output from the mode lockinglaser diode device enters the semiconductor optical amplifier.
 14. Alight output device comprising: a laser light source; a semiconductoroptical amplifier according claim 1; an alignment device that adjusts arelative position of the semiconductor optical amplifier with respect tolaser light entering the semiconductor optical amplifier; and asemiconductor optical amplifier control device that controls operationof the semiconductor optical amplifier.
 15. The light output deviceaccording to claim 14, further comprising a mirror and a light isolatorpositioned such that laser light output from the laser light sourcepasses through the light isolator and then is reflected by the mirror.16. The light output device according to claim 15, further comprising ahalf-wave plate and a lens positioned such that laser light reflected bythe mirror passes through the half-wave plate and lens and then entersthe semiconductor optical amplifier.
 17. The light output deviceaccording to claim 16, wherein the light isolator is positioned toprevent light returned from the semiconductor optical amplifier fromreturning to the laser light source.
 18. The light output deviceaccording to claim 17, further comprising an output lens, wherein thelaser light is optically amplified in the semiconductor opticalamplifier, and is output outside the system through the output lens. 19.The light output device according to claim 14, wherein the laser lightsource is a mode locking laser diode device, and pulse laser lightoutput from the mode locking laser diode device enters the semiconductoroptical amplifier.
 20. The light output device according to claim 14,wherein the reflecting mirror, the half-wave plate, and the lens are onthe alignment device.
 21. The light output device according to claim 20,wherein the alignment device comprises an XYZ stage and when thethickness direction of the laminated structure in the semiconductoroptical amplifier is the Y direction and an axis line direction of thesemiconductor optical amplifier is the Z direction, the reflectingmirror and the lens are moved in the X direction, the Y direction, andthe Z direction by the alignment device.
 22. A light output devicecomprising: a laser; a semiconductor optical amplifier that amplifieslight output by the laser; a light isolator between the laser and thesemiconductor optical amplifier; a mirror between the light isolator andthe semiconductor optical amplifier; a half-wave plate between themirror and the semiconductor optical amplifier; and a lens between thehalf-wave plate and the semiconductor optical amplifier, wherein, (1)the semiconductor optical amplifier comprises (a) a laminated structureincluding (i) a first compound semiconductor layer that has a firstconductivity type and is composed of GaN compound semiconductor, (ii) asecond compound semiconductor layer that has a second conductivity typedifferent from the first conductivity type and is composed of GaNcompound semiconductor, and (iii) a third compound semiconductor layerthat has a light amplification region composed of GaN compoundsemiconductor, the first compound semiconductor layer, the thirdcompound semiconductor layer, and the second compound semiconductorlayer are sequentially layered, (b) a second electrode on the secondcompound semiconductor layer, and (c) a first electrode electricallyconnected to the first compound semiconductor layer, (2) the laminatedstructure has a ridge stripe structure, and (3) a carrier non-injectionregion is provided in the laminated structure, the carrier non-injectionregion extending from the light output end face of the ridge stripestructure and towards the light incident end face of the ridge stripestructure along an axis line of the semiconductor optical amplifier, thefirst, second, and third compound semiconductor layers being present inthe carrier non-injection region, and the second electrode not beingpresent in the carrier non-injection region.
 23. The light output deviceof claim 22, wherein, for the semiconductor optical amplifier, when awidth of the ridge stripe structure in a light output end face isW_(out) and a width of the ridge stripe structure in a light incidentend face is W_(in), W_(out)>W_(in) is satisfied.